FIRING INFORMATION

Kiln Wash/Cone Temp.Chart/Firing Glazes

Kiln Repair & service For southern California contact us at 818/782-1500

Kiln Wash directions Kiln wash is applied to kiln shelves to protect them from glaze drips. On a washed shelf, drips can be easily removed without gouging or marring the kiln shelf. Mix the kiln wash with water to a thin cream consistency. Apply only one coat at a time. Use a wide paintbrush or utility brush (a 2" - 3" wide, soft bristle brush generally works best). Three individually fired on coats of kiln wash are preferable. APPLYING KILN WASH 1) Make sure the floor of the kiln and the tops of the shelves are coated with kiln wash. This will protect these surfaces from melting glaze and ceramics. 2) Do not coat the bottom or sides of the shelves. 3) Do not apply kiln wash to the brick sides or element holders. 4) Apply the kiln wash to the thickness of a post card. 5) The only purpose of kiln wash is to prevent any glaze that drips from a piece from sticking to the floor or shelves. This saves both the piece and the floor or shelves. If dripping should occur, simply remove dripping and cover the spot with new kiln wash. 6) When you are applying kiln wash to your shelves for the first time, it helps to dampen the top of your shelves with a wet sponge or a water-filled spray bottle first. This makes the kiln wash go on easier and more evenly. 7) For the kiln wash to really protect the kiln shelves it is best to apply three separate coats. In addition it is best to fire each coating separately. (If you brush one coat on, let it dry and then brush on another, you could actually be brushing off the first in the process, so ideally each coat should be fired on). The shelf can be used while firing the kiln wash on, so theoretically you would put one coat on, load the shelves and do your test firing of the kiln. The second coat would be fired on in the first bisque and the third coat in the second bisque or first glaze (whichever comes next). Fire at least to a minimum cone 018 or hotter. Hot enough to give the kiln wash enough adherence to the shelf to prevent it from coming off in the second coating. Note that some people get away fine without three firings of the kiln wash. However, we include this recommendation as a “best practice”. 8) If you notice that your kiln wash is flaking off, use a paint scraper (or something similar) to remove any loose bits, then reapply kiln wash. If glaze drips onto your shelf, use the paint scraper to pop the glaze drip off and clean up any loose areas around the area, then dab some more kiln wash in the bare area.  Be sure to store shelves with the sides that have kiln wash facing each other only. Otherwise pieces of kiln wash can fall on glaze ware causing damage to them.IMPORTANT: Also be sure not to let kiln wash touch your heatind elements. This will cause them to fail.   Cone Temperature Chart Please note: Due to malfunction, kiln firing temps may vary in the different sections of your kiln. All kilns are not created equal. Different cones & bars                                                                           From left to right. Bar, cone, large cone & self supporting or witness cone. For ceramics, kilns are not fired just to a temperature.  They are fired to a "cone" level, which accounts for time as well as temperature.  Think of it as heat absorption rather than just temperature. The middle self-supporting cone is perfectly fired.  The left is over-fired, the right is under-fired. Cone's come in different numbers, each of which corresponds to a heating-rate/temperature-combination which will make that cone deform.  At the beginning of the firing the cone is standing at an 8 degree angle.  A perfectly fire cone will be bent to a 90 degree angle.  If the cone is bent less, the kiln was under fired.  If the cone is bent more, the kiln was over fired. Notice the way the cones are numbered.  The hottest is 10, going downward to 1.  Then as it continues to getting cooler it continues at 01, 02, etc.  So there is a difference between cone 5 and cone 05!

Cone number	Orton Cones Final temp  in degrees F at ramp rate of 27 degrees F/hr	Orton Cones Final temp  in degrees F at ramp rate of 108 degrees F/hr	Orton Cones Final temp  in degrees F at ramp rate of 270 degrees F/hr	In standard firing, cones of the right number are placed around the kiln and are watched.  When the cones fall the kiln is turned off.  This works the same for gas and electric kilns. Electric kilns with electronic controllers (such as the Skutt KilnMaster series, Cress) have a thermocouple which continuously measures the temperature, records it over time, and shuts off the kiln when the appropriate heat absorption has been met.  So if the ramping temperature is fast, the kiln will go to a higher temperature before it turns off than if the ramping temperature is slow (thus allowing the clay to absorb more heat along the way.)  The final temperature is most affected by the rate of temperature increase over the last 300 to 400 degrees of firing. Note:  Think of the 0 in a cone number as meaning "minus".  So 06 is much cooler than 6 because it is like a "minus 6".
10	2284	2345	2381
9	2235	2300	2336
8	2212	2273	2320
7	2194	2262	2295
6	2165	2232	2269
5	2118	2167	2205
4	2086	2142	2161
3	2039	2106	2138
2	2034	2088	2127
1	2028	2079	2109
01	1999	2046	2080
02	1972	2016	2052
03	1960	1987	2019
04	1915	1945	1971
05	1870	1888	1911
06	1798	1828	1855
07	1764	1789	1809
08	1692	1728	1753
09	1665	1688	1706
010	1636	1657	1679
011	1575	1607	1641
012	1549	1582	1620
013	1485	1539	1582
014	1395	1485	1540
015	1382	1456	1504
016	1368	1422	1465
017	1301	1360	1405
018	1267	1252	1283
019	1213	1252	1283
020		1159	1180
021		1112	1143
022		1087	1094

Trouble Shooting

 

While we would love to be able to prevent all mishaps, understanding the why's and how's they come into being will help you diagnose and hopefully, prevent problems in the future. Like everything else though, a certain amount of failure should be expected. We nor our materials are perfect all the time.

1. Did you get a good fire? The fact that you set your digital controller to cone 06 does not insure that is what your kiln did. We recommend that you use self supporting witness cones in every fire. Witness (shelf) cones are used in a series of three:

• Guide Cone is one cone cooler than the desired temperature.
• Firing Cone is the cone temperature desired.
• Guard Cone is one cone hotter than the desired cone.

Use cones in each fire and on every shelf. If you simply cannot use all three sizes, use the firing cone on every shelf, preferably two. Keep a record of your results and you will discover your kiln’s personality- hot spots, cold spots, etc. By using witness cones and verifying that your kiln fired successfully and the desired cone was reached, one important variable will be eliminated as a problem source.

2. What type of glaze was used? Knowing the type of glaze that was used and how it was applied may give us a clue.

3. What does the surface look like? Holes, bare spots, fine lines? Following is a description of the problem, possible causes and potential solutions:

Bare Spots in glaze surface:
called crawling.

Problem 1:
Often caused by dirt, dust or oil, prohibiting glaze from adhering to the ware.

Solution:
Make sure piece is clean before painting by wiping with damp (not wet) sponge. Make sure hands are clean of oils, hand lotions, etc.

Problem 2 :
Too heavy of an application.

Solution:
Lighten application. Lightly touchup bare spots and re-fire.

 

Small holes in glaze:
called pinholing.

Problem:
Underfired bisque is continuing to mature during glaze firing, causing emission of gasses and interrupting the glaze surface.

Solution:
Bisque should be fired to 03-04 and glazes two cones cooler to prevent ware from reheating and releasing gasses. Sometimes repeating fire can smooth the glaze.

 

Large Pinholes: also called craters, fisheyes or bubbles.

Problem:
caused by gases escaping during firing due to
a. underfired bisque
b. firing glaze while wet
c. ware not fired hot enough or piece was fired/cooled too fast.

Solution:
a. Bisque should be fired to shelf cone 04 or hotter. Two cone difference between bisque fire and glaze fired necessary.
b. Allow glazes to dry 24 hours prior to firing.
c. Fire glazes to shelf cone 06 or recommended cone. Glazes need an even steady ramping of temperatures and cooling. File down craters, reglaze and refire.

 

Glaze rolls back: called flip back

Problem:
a. Glaze was applied too heavily, without sufficient drying time between applications.
b. Piece was fired too wet.
c. Oil or grease on piece

Solution:
a. Allow glaze to dry between coats, apply thinner coats.
b. Allow piece to dry 24 hours before firing.
c. Make sure surface of bisque is clean of dust; wipe down with damp sponge. Sometimes flip backs can be corrected by sanding down, reapplying color and refiring.

 

Fine lines in glaze surface after firing: called crazing

Problem:
Generally caused by glaze and body fit problem. When talking technical, will be described as a coefficient of expansion problem.

Solution:
Use mature bisque. Some glazes, especially non-toxic formulations, are more sensitive to application.

 

After firing, glaze falls off pieces:
called shivering

Problem:
Incompatability between ware and glaze.

Solution:
Coefficient of expansion. Body shrinks at a different rate than the glaze, causing the glaze to fall off. Be very careful as the pieces of glaze can be quite sharp. Some glazes, particularly non- toxic glazes, are not as accommodating of less than perfect bisque.

Gloss glaze is matte

Problem:
Glaze did not reach maturation during firing.

Solution:
Use witness cones to make sure proper fires are achieved. Refire piece.

Piece cracks in kiln: called “a mess”

Problem:
a. Thermal shock or expansion of two or more glazes
b. Too heavy of glaze in bottom of piece.

Solution:
a. Use same type of glaze inside and out when glazing. Recommended to use gloss glaze inside pieces, even when mattes are used on outside.
b. After rolling glaze on inside of piece, invert piece to drain excess glaze.
 

Loading a Kiln for Best Results Loading a kiln for firing is not a simple matter of placing shelves and stacking ware. The more thought and planning that is put into loading, the better the results. Ware and shelf placement, the size of the load, the firing characteristics of the kiln and the type of ware being firing are all important factors. First the Furniture Kiln shelves come in all shapes and sizes. For economy of space, it is best to choose shelves similar in shape and size to your kiln chamber. For instance, use a round or multi-sided shelf in a round or multi-sided kiln. Keep the size small enough so there is at least 1" of space between the shelf edge and the side of the kiln or the Kiln-Sitter. Also allow some room between the top of your ware and the lid of the kiln and leave space for witness cones amongst your ware. Select posts in heights to accommodate the ware you are firing. Leave some room between the kiln shelves for air to flow, for heat transfer and for removal of fumes. Half shelves are very useful to improve air movement in the kiln. Use two side by side with a 1/2" space between them and you don't lose much stacking space. Some kiln manufacturers recommend placing shelves directly on the floor of the kiln. Most suggest using 1" posts to put the bottom up from the cooler floor. This creates an insulating layer much like a storm door. Setters and Stilts Air movement in the kiln is clearly a big consideration - one of the most important when loading a kiln. Ceramics need to heat uniformly to prevent warping and stresses in the ware. Air needs to move around shelves and around individual pieces. Plates and tiles benefit from the use of tile and plate setters or stackers. Shelf style setters allow air to move under the large flat objects so they heat more evenly. Avoid heating large flat objects directly on the cooler shelf. If you are firing decorated tiles or plates, vertical setters economize on space, and sets can be stacked to fit even more. Glazed ware needs to be stilted or dry footed or the melting glaze will stick the ware to the kiln shelf, ruining both. Stilts also provide space for air to move around all sides of the ware. Porcelain and stoneware can not be stilted. The stilts embed into the ware during firing. Instead, use high fire kiln wash or silica sand on the shelf. Use prop to prevent sagging of porcelain. Consider Heat Distribution It is important to evaluate heat flow in your kiln and to make this a consideration in loading. Use pyrometric cones to determine the heating characteristics of your kiln so you know where the hot and cooler places are. Arrange your ware with different sized pieces on the same shelf to allow better heat flow. Don't Overfill Perhaps one of the most important factors in good fired results is enough air to mature the ware - to burn out organics in bisque and develop best colors in glazes. Shelf and ware placement and the use of setters and stilts can all help this, but here are a couple more tips: When stacking bisque, invert bowls and mugs opening to opening instead of nesting- this helps air move around all sides of a piece and prevents black rings and spots in the bottom of ware. Fire bisque lids and bottoms together. To get the best fit for lids, fire them on the piece they match. This will let the two pieces shrink together so you get a good tight fit. Fire all glaze pieces separately. Leave space between ware - don't overfill. There is a temptation to cram as much as possible into the kiln to economize on firing costs. Ware fired too closely together creates firing problems. If you must overfill, fire very slowly and vent adequately. Mix thin and thick-walled pieces together throughout the load don't concentrate them in one area where they are competing for air and heat. Use downdraft venting to move air through the kiln and to remove fumes created during firing.

Firing Handbuilt or Thick Cast Ware   Most pinch pots, coiled or slab built ware generally have thicker walls than their slip cast cousins, although molded pieces may be cast heavily as well. With these types of pieces, the thicker walls create some unique challenges for firing. Basic problems that can occur when firing handbuilt or thick cast ware include cracking (or exploding) and carbon burnout. Because of the thicker walls it is important to fire slower and control heating and cooling during firing. Preparation of the piece is important as well. During forming, stresses within the piece may result in hairline cracks that appear during firing. It takes longer to fully dry a thick piece. Uneven drying can result in warping or cracking. For pieces properly prepared, handled and dried, the next critical step is firing. Firing Issues Is the ware fully dry? Ware that is not adequately dried will crack or explode during the early stages of firing. Water inside the pores of the ware turns to steam, exerting pressure inside the ware. To fully dry a thick walled piece, the ware needs to be warm for more than 12 hours. Am I firing too fast? All bodies expand when heated and shrink when cooled. If the outside wall expands more than the inner wall, stresses occur. If these stresses are large enough, they pull the body apart and cause cracking. A 1" thick wall can have more than a 10° F difference in temperature between the hotter and cooler surfaces. Firings need to be slowed down for thicker wall pieces. Likewise, it is important not to cool too fast. Have I allowed enough time for carbon burnout? It is important to burn out all carbon from the ware before higher temperatures are reached (1200° F or 650° C). It takes time for oxygen to move into the pourous body, react with the carbon and then leave. If carbon remains, many problems can occur. These include problems with colour, glaze fit, strength, blistering and discolouration. Use of a downdraft vent system, combined with slower heating, virtually eliminates carbon related problems. Heating & Cooling Control The best way to control cracking problems during firing is by controlling the rate of heating and cooling for the kiln. During firing, materials that make up the body undergo many changes. Special care must be taken at temperatures below 1500° F (815° C) to heat the body uniformly. Remember, the thicker the wall, the slower the heating should be done. Above 1500° F temperatures can be increased more rapidly because the changes are less likely to causes stress cracks within the ware. What Kind of changes occur? All clays can many minerals contain water which does not leave the body until above 700° F. Organic (carbon) materials need to be oxidized (burned out). Other minerals, such as calcite, break down and give off a carbon dioxide gas. Minerals such as flint (silica) undergo a sudden expansion on heating to 1060° F and contraction during cooling. How can I control my heating? This depends on the controls for the kiln. With switches, leave them on medium settings longer. It should take more than 3 hours to reach red heat and even longer for thick pieces or a heavily loaded kiln. Make sure the kiln is well vented below red heat and closed up completely above red heat. Keep the kiln closed during cooling for 8 hours or until well below red heat. When did cracking occur? Often the crack itself can be examined to determine when it occurred. If the edges are sharp, then it probably occurred during cooling. If the edges are rounded or if the glaze has flowed into the crack, then it occurred during heating. What else can cause cracking? Uneven heating is a primary culprit that causes cracking during firing. Hot and cold spots in the kiln can cause uneven heating of pieces. Use witness cones to diagnose hot and cold spots and then adjust the switching or use a downdraft vent to help even out the heating. Careful loading of the ware in setters and on stilts can also help heat circulate around the piece. Underfired bisque is not as strong and may crack more easily during the glaze firing. Use witness cones to assure a proper firing and prevent underfired bisque. Gas expanding in air pockets which developed in the ware during forming can cause large cracks during firing.

 

Firing Red Glazes

Red glazes are among the liveliest, brightest colors we can use, but unfortunately, red glaze problems are legendary. Many of us simply give up using reds or accept whatever results we can get, including the problems. Common Red Glaze Problems   improper color development- dark bluish or purple cast to the glaze color loss- glaze looks gray, white poor surface texture- a rough matte finish and/or visible surface defects "The Strawberry Effect"- tiny black dots or spots in the fired glaze crazing- a crackled or cracked appearance in the fired glaze Some of these problems relate to the preparation of the piece and application of glaze, but many defects are the result of improper firing practices. Preparation and Application   ware must be clean and free of dust do not apply red glaze to greenware apply only to properly fired bisque (use witness cones to verify firing) work area and tools should be kept clean and free of contamination no eating/smoking in glazing area glaze away from cleaning areas apply adequate coats of glaze - four is often recommended allow each coat to dry How Colors Develop Many ceramic glazes need to be fired in an oxidizing (air) atmosphere for best results. Red, orange and yellow glazes in particular are very oxygen sensitive. This means they require sufficient air during the firing to bring out the colors to their fullest and to prevent surface/ finish defects. Firing reds requires us to control the firing rate and properly vent the kiln. Controlling the Firing Rate Nearly all ceramics fire better when fired slowly below red heat. Slow firings have the advantage of allowing the necessary physical and chemical changes to occur in the ware. Slower firings also permit time for sufficient air to enter the kiln and displace the carbon monoxide. This is true for both bisque and glaze firings. Firing rate can be controlled using the settings on an automatic kiln, programming an electronic controller or by adjusting the switching. Control or slowing of the firing rate is most important in the early stages of the firing when most of the reactions are occurring and when air is needed to burn out the organics in ceramic materials. Near vitrification (the end of the firing) a faster rate is desirable and can usually be applied. Venting for Proper Air It is most important that enough air gets into the kiln in the early stages of firing. This is when the organic materials are burning out of the ware and air reacts with carbon to form carbon monoxide. Kilns can be vented manually or with an automatic venting system. Manual Venting Manual venting lets the fumes out of the kiln, but is only somewhat successful at letting air into the kiln. For manual venting, the top lid should be propped and the peephole plugs out for at least the first hour and a half. Slower firings require additional time. When the kiln reaches red heat, the lid can be closed and peephole plugs replaced. Leaving the peephole plugs out for the whole firing is not recommended since it can cause cold spots in the kiln. Manual venting works better with a smaller load. Also, using split level shelves allows air circulation and helps ventilation. Manual venting is recommended whenever a downdraft vent is not available. When venting manually, it may be desirable to locate red glazes on the top shelf to assure sufficient air. Automatic Downdraft Venting A downdraft automatic venting system like the Orton KilnVent efficiently brings the proper amount of air into the kiln and removes the fumes for exhausting. The kiln lid and peepholes remain closed the entire firing. Using the Orton Vent, test have shown reds can even be fired with other colors with good results. Firing to Proper Cone Number Using witness cones on the kiln shelf to verify results is important to good results. Many problems occur when red glazes are not fired to the proper cone number. Blistering can occur if underfired and loss of color if overfired. Glaze on underfired bisque may craze. Firing lead free glazes to the proper cone number is especially important. Firing reds can be a challenge, but by following good preparation, application, firing and venting practices, and by firing to the proper cone number, most problems can be eliminated.

Glaze Defects and Corrections

 

1.BISQUE AND GLAZE FIRINGS

On manual controlled kilns the total time to reach both bisque and

glaze firings should be at least ten hours.

• Always load the glaze kiln as densely packed as possible with pots.

If there is not enough ceramic ware to fill the kiln place posts and

shelves in the kiln to obtain a slow even firing.

• In the bisque firing the ware can be stacked together with pieces

touching. In the glaze firing the ware should be physically separated.

 

 

COMMON GLAZE DEFECTS

Listed are the most common glaze defects. Often if a defect can be

correctly identified a suitable correction can be enacted with a successful

result. One of the first stages in determining the cause of any

cracking problem is to find out when the cracks started in the ceramics

process. By carefully examining the type of crack (sharp or round

edge) it will indicate if it occurred before or after the glaze firing.

Determining when a crack started is a significant step in correcting a

defect in future ceramic pieces.

Round Edge Crack

A round edge crack has developed where the glaze rolls back from

the edge of the crack. Round edge cracks occur when the bisque is

already cracked and the glaze is applied over the crack. Some cracks

in the bisque are very small and go unseen however; once the glaze

is applied and the bisque is fired the crack increases in size.

Prevention:

• do not stack bisque in storage, do not drop bisque

• Inspect all bisque before glazing

Sharp Edge (hairline crack)

A sharp edge crack in the glaze fired piece indicates it occurred after

the glaze had “set” and became hard in the glaze firing process.

When this type of crack occurs it indicated a bisque and glaze not

fitting correctly.

 

2. GLAZE DEFECTS/CAUSE & CORRECTION

At some point there will be a defect when firing either clay or glazes

however, bad clay or glaze results can be kept to a minimum by

understanding ceramic materials. It is most important to diagnose

the problem quickly and apply a solution in order to fulfill orders and

insure good customer relations. The bisque products that you are

using in your studios are formed and fired to the highest standards.

We recommend examining all shipments of bisque products and

report any discolorations or irregularities in the bisque surface. Keep

all bisque clean and dust free before glazing. Often a few simple

guidelines will greatly improve the look and finish of the ware.

 

GLAZING AND FIRING RECOMMENDATIONS

Many glazing and firing problems can be avoided by following a few simple guidelines.

 

• The bisque should be clean and dust free to insure a compatible

glaze fit.

• The underglaze and glaze application thickness should not exceed

the width of a dime.

• To prevent damage to kiln shelves use a light application of the

recommended kiln wash.

• When glazing the bottom of ceramic pieces stilts (star shaped ceramic

supports which are used under the ceramic form) can be used to keep

the ware from direct contact with the kiln shelve. The ware can also

remain unglazed on the bottom and placed directly on the kiln shelve

without the use of stilts.

• All clay objects must be thoroughly dry before the bisque firing.

Clay contains mechanical and chemical water, which must be slowly

released during the first stages of the firing. On computer controlled

kilns always use the SLOW or MEDIUM  firing speed mode.

Correction:

• Try firing the glaze kiln one cone higher or lower to promote a

compatible glaze fit.

• Glaze the inside and outside of the bisque to equalize glaze fit.

• Use another glaze manufactures product, which might fit the

bisque compatibly.

• Often a thinner glaze application will stop a sharp edge crack.

• Slow cool the glaze kiln.

 

3. CRAZING

 

Crazing is one of the most common glaze defects luckily; there are

several corrections, which can eliminate crazing.

Glaze Crazing

Crazing is a fine network of lines in the fired glaze surface. Glaze crazing

occurs when the glaze is under too much tension as it cools in the

kiln. The bisque and glaze are not fitting upon cooling.

Corrections:

• A longer glaze firing often corrects glaze crazing. Try firing the kiln to

cone 06 in at least 14 hours.

• Firing one or two cones higher also improves glaze fit. Try firing the

kiln to cone 04.

• A thinner glaze application can stop crazing.

• Using another manufacturers glaze can stop crazing.

Glaze Shivering

Shivering looks as if “paint chips” are flaking off of the fire glaze.

Shivering is the opposite from crazing. In shivering the glaze is under

too much compression when cooling on the pot. Again the bisque

and glaze are not fitting upon cooling.

Corrections:

• Using another manufacturers glaze can stop shivering.

• A thinner glaze application can sometime stop shivering.

• Firing the glaze kiln one or two cones higher or lower can stop

shivering.

Glaze Crawling

When a glaze crawls it rolls back on itself sometimes exposing a bare

spot in the underlying bisque. Crawling can occur when one glaze

overlaps another or a glaze application is too thick. If the raw unfired

glaze is cracked or extremely dusty it can crawl in the glaze firing.

Corrections:

• Apply a thinner application of glaze (most glaze applications can be

applied thinner than the width of a dime).

• When overlapping glazes a thin application for both glazes is

required.

• Dusty or dirty bisque can cause the glaze to crawl. Always cover

bisque pieces until glazing.

• Some glaze are more likely to crawl and simply choosing another

glaze is the best solution.

 

4. Glaze Blistering

 

There are several possible causes of glaze blistering. Some glazes

when over fired can run and blister. A blister defect has a sharp crater

edge. Another cause of glaze blistering occurs when the glaze kiln is

fired at too fast a rate of heat increase. The immature glaze can blister.

If a glaze has been fast fired it can often be placed back into a

glaze kiln to remove the blisters. Re-firing cannot remove blisters

from over firing.

Corrections:

• If the blister is caused by a fast glaze firing re firing the piece will heal

the blisters.

• A longer time to the recommended glaze firing temperature is

required with new and re fired glazes.

• The kiln should be fully loaded with pots, if there are not enough pots

to fill the kiln place posts and kiln shelves to achieve a dense stack.

• If the blister is caused by over firing new glazes should be fired one or

two cones lower.

Fast Glaze Firing

Some glazes when fired too fast will have bubbles suspended inside

the glaze. Often they block out a color underneath the glaze. Many

glazes go through a stage where they bubble during the firing.

Bubbles in the glaze are more apparent in clear, transparent glazes.

Glazes need increased time to reach their maturing temperature and

become clear and smooth.

Corrections:

• Try a slow glaze firing to the glaze maturing temperature.

• Often firing one cone higher will remove bubbles from a glaze.

• Re firing a glaze will give it more “heat work” and remove bubbles.

• Stacking the glaze kiln tightly will increase the thermal mass during

the firing.

• A thinner glaze application will stop bubble held within the fired

glaze.

Glaze Run/Drip

Glazes can sometimes run or drip under certain application, and firing

conditions. The drip can cause the glaze and underlying bisque to

stick to the kiln shelf. If the bisque cannot be removed from the shelf

try using gentle pressure with a chisel placed at an oblique angle

between the stuck glaze and kiln shelf. In some situations where one

glaze is applied over a second glaze it can cause both glazes to run.

Always test when applying one glaze over another.

Corrections:

• Use a thinner glaze application (most glazes should be applied slightly

thinner than the thickness of a dime).

• Fire the kiln one or two cones lower.

 

5. Crystal Glaze

 

Certain glazes can grow crystals upon cooling (devitrification) in the

glaze kiln. The crystals frequently look like white specks in the glaze,

which can cause the glaze to change color and opacity in random

areas.

Corrections:

• Cool the kiln at a faster rate.

• Stack the kiln with more space between pieces.

• If a glaze grows crystals in a particular part of the kiln use other glazes

in that section of the kiln.

• Often a similar glaze effect can be achieved by using another glaze

that does not grow crystals.

 

SUMMARY

While no body of information and advice can cover every possible

glaze defect a majority of problems start with the glaze application

being too thin or too thick. Fast glaze firings with kilns not packed

with pots, shelves, or posts are the next statistically proven areas for

glaze defects which include, pinholes, blisters, and changes in glaze

color/texture. The simplest advice, which is sometimes hard to follow,

is to test every glaze before committing a great deal of time and

effort into a project. An accurate procedure for glaze testing should

entail using the glaze on at least three vertical pieces placing a test

on the bottom, middle and top kiln shelves. It is important to protect

the kiln shelf under each test piece. It can cause both glazes to run.

Always test when applying one glaze over another.

Question: What is hotter?  Cone 6 or cone 06?

Answer: cone 6 is much hotter. Cone 6 is considered high fire and is well over 2000 degrees. Cone 06 is low fire and is in the 1800's. Cones go in this order.

Most popular sizes from cooler to hotter:                             ;       06, 05, 04, 03, 02, 01, 5, 6, 10

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Tip: When working with kiln fire clay always let dry in room temperature. Don't rush the drying process by putting in an oven or in the sun. This will cause rapid shrinkage and cause cracking and curling of the clay. To avoid exploding clay, keep thickness at 1/2" or less. If the piece is a three dimensional item hollow out the piece to no more than 1/2" thick. 

Note: The slower the firing speed is, the thicker the project can be with less risk of blowing up. 

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Problem: Do you Have a brush that is bent to one side and its tough to use?

Repair: Wet the brush with water and smooth out till straight. Then place it in a freezer for about a week. Take out defrost and it should be straight again.

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Problem: So you glaze fired some low fire earthenware and the glaze came out very cloudy....

Repair: This could have been caused by a couple of different reasons. First, kiln was not hot enough. Place self-supporting witness cones for the cone temp your firing on the edge of the shelve and re-fire. Second reason could be too much clear glaze.

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Problem: Do you have small sharp glaze pinholes in your glazes item after glaze fining? You most likely have "inmature bisque". That comes from the fact that when your item was greenware and was bisque fired, it was under fired (not to proper temperature). When you glaze fired, all of the gasses that didn't burn off during the bisque firing are trying to do so during the glaze firing and push through the glaze creating the pin holes and bubbles. 

Repair: Carefully without cutting yourself and wearing protective eyewear, grind down the bubbles with a dremmel or a stilt stone. Then dab a little more clear glaze over the exposed areas, let dry and re-fire. It doesn't always work but the odds are on your side.

Cracking and Thermal Shock   Cracks that appear in fired ware which were not caused by casting or drying problems may be the result of thermal shock. Thermal shock occurs when too much stress is created in a piece of ware during the heating and cooling process. It comes from temperature differences in the ware and can cause small to large cracks in the piece, or the piece may actually break. Why Does Cracking Occur? The tendency of a piece of be susceptible to thermal shock is related to: the strength of the piece the thermal expansion of the material Thermal shock can result when changes in temperature occur in the kiln during heating and cooling. As temperature changes rapidly, the outside of the ware and kiln furniture becomes much hotter or cooler than the inside. This causes stresses which may result in cracking or breaking. The following can effect thermal shock: a fast heating rate or rapid cooling a sudden influx of cool air such as opening the kiln lid when the kiln has not finished cooling in a gas kiln - turning off the gas and allowing cool air from the burners to enter the kiln Thermal shock can also occur when ware is stressed in use, such as a casserole or dish that is taken from the freezer or refrigerator and put into a hot oven. The stronger ware is, the better able it is to resist cracks due to thermal shocking. Weak ware will be more likely to break when stressed. A piece that is porous will also be weaker, making it easier to crack. Water or condensation that enters pores in the ware can turn into steam and expand and this can cause cracking when heated. The harder (hotter) ware is fired, the less porous it will be. Ware that expands and shrinks a great deal during heating and cooling is also more likely to be affected by thermal shock Most kiln shelves contain cordierite because this material has a lower expansion than most of our ware and so less affected by thermal shock. What Happens to Ware During Firing? During heating and cooling, the body and glaze undergo many physical and chemical changes. Some of these include: moisture is driven out of the ware if this occurs too rapidly, cracking can occur organic material is oxidized and released from the material the glaze softens, melts and flows during heating and may trap gas the body expands as it is heated and contracts during cooling the glaze solidifies and contracts during cooling If the body or glaze contains silica, it will expand rapidly at 1063° F on heating and contract during cooling. If the heating or cooling is rapid near this temperature, this change can lead to cracking of the piece. Control of heating and cooling is especially critical when firing thick-walled pieces or pieces with an irregular wall thickness. Reducing Thermal Shock There are several easy ways to minimize the potential for thermal shock: use a smooth, moderate heating rate let the kiln cool naturally with the lid closed use a controller to slow down the cooling time avoid sudden temperature changes A programmable controller such as the Orton AutoFire™ is the best solution to control the heating and cooling rates and to get a smooth temperature rise. If instrumentation is not available, heat loss during cooling can be controlled to some extent by keeping the kiln closed until well below red heat (900° F). To be sure that ware is properly matured, be sure to use witness cones. Underfired bisque will continue to shrink during the glaze firing and this can result in a poor glaze fit.

Cracking and Warping Caused by Drying and Casting   In some instances cracking and warping problems share a common source: the casting and drying of the piece. In other cases, cracking may be related to how the piece is fired. This Tip looks at problems related to casting and drying. Drying Ceramics Ceramics contain clay which can absorb and hold water. Before firing, it is important to remove all of the physical water so that the piece will not crack or explode when heated. This is often accomplished in steps with firing being the final stage. During firing, the chemical water is removed from the piece and it gains strength while developing physical surface characteristics. Understanding Drying Simplified, drying is the removal of water from body by evaporation. As the ware is dried, the film of water separating the clay particles gets thinner and thinner, the solid particles get closer together and the piece shrinks. Shrinkage stops when the particles finally contact each other. Drying Faults Cracking, distorting and warping are problems that may not become evident until after firing. They are usually caused by drying too fast or unevenly. If ware is heated too fast, the pressure from water vapor inside the piece can cause cracking. Ware dried only on one side can shrink more on that side causing warping or bending of the somewhat plastic (flexible) piece. When one surface finishes drying, the piece is now too stiff to recover and the warping becomes permanent. This can lead to cracking. Bodies made of very plastic clays or compositions having a high clay content require attention to uniform, slow drying. Thicker walled pieces will often have a greater tendency to warp or distort. Care needs to be taken to allow for uniform air movement around all sides of a piece to avoid drying problems. Sometimes drying must be slowed down to avoid cracking. Handles on cups can have a tendency to pull away from the mug. Doll heads and chest cavities may deform inward. Reducing Warping and Cracking To reduce warping and cracking, take steps to dry more slowly and more evenly from all sides. Don't dry a flat object on a wet or cool surface like a formica or plastic table top or damp newspaper. The piece can only dry on one side. Instead, dry objects on something porous like wood or plaster or set them so air can circulate around them. If necessary, turn pieces over during drying for more even result. Slow the drying of thick walled pieces and hand built ware. Support areas during drying that might cause stresses to build up. Drying Techniques slip cast ware - may warp or crack if stressed (deformed) when removed from the mold. Even if the ware is gently returned to the original shape, the created stress will ultimately cause the piece to warp or crack. wheel thrown ware - should not distort during drying unless subjected to further mechanical forces - let the ware dry naturally on a bat or shelf and it should be fine. thick handbuilt ware - needs to be dried for a very long time before it can be fired or it may explode during firing. Several days may be required or a low heat drying in an oven may be necessary to remove all the water. plates - even drying is particularly important with plates. Warping can cause the center of plate to fall or arch up. Rims and centers must dry evenly to prevent warps, humps and cracks. drying tiles - drying tiles can present a particular challenge because it can be difficult for the piece to dry evenly. Usually air is passed over the top of the tile. This results in warping because the bottom of the tile remains wet. Drying tiles in tile racks can help air movement for more even drying.

Firing Ceramic Bodies   Most bodies and glazes contain clay. These fine clay particles give the body and glaze many desired properties and bonds other materials together. When the body is fired: clay and other minerals in the body start to change clay/minerals break down and react with other materials to produce gases at 900° F (red heat), tightly held water molecules begin to break free and leave gases such as sulfur oxides and some fluorine may be released as the temperature increases, clay and other minerals continue to change and react with each other to form new compounds that will be part of the final product some products form glass which will bond everything together Gases The gases which form need to be removed from the body. For example, carbon is in the clay and organics are added to the body, glaze or decoration to improve strength during handling or application. These must be removed during firing to avoid defects. Firing Conditions Firing conditions can also determine many properties of the fired product. Firing too fast at lower temperatures may not allow sufficient time for materials to react and gases to leave the body or glaze. Firing too fast can result in weaker bodies pinholing bubbling of the glaze color changes in the body color changes in the decoration mildewing of porcelain crazing or peeling of glazes if body is not properly mature Earthenware typically fired from Cone 07 - 03 made with talc, less expensive clays clays contain many impurities, need fired longer at lower temperatures low shrinkage porous after firing usually tan or red in color frequently glazed or stained sometimes used as-fired Often, problems arise because bodies are underfired. The piece may look okay, but is porous and weak. Also, underfired bodies may not match the expansion of the glaze used in a later firing. This can result in glaze fit problems or cracking of the body in use. The high iron and carbon content of these clays requires plenty of air during firing to maintain good color and to burn out all of the carbon. If this is not done, many problems can occur when the product is glazed and refired. Stoneware typically fired between Cone 6 - 10 large number of compositions contain clays and other minerals with many impurities, including sand, feldspar and grog additives are used to provide plasticity, workability, strength, color, and to reduce shrinkage colors depend on raw materials Because of the additives and impurities, care needs to be given to how stoneware is fired and to proper ventilation of the kiln early in the firing to burn out organics. Stoneware is vitreous and contains a high percentage of glass in the fired product. For color variations, mature the ware under reducing conditions. Porcelain typically fired from Cone 3 - 10 compositions vary, but contain high quality materials colorants may be added bodies are hard, white, translucent very high glass content narrow firing range - need to be fired close to slump or sag point for best fired properties. Because color is very important, these bodies need to be fired with plenty of air below red heat to be sure all the carbon is removed. Shrinkage is high and special care must be given to supporting porcelain during firing or it will warp and distort. Critical Firing Periods For all clay containing bodies and for most glazes and decorations: be sure ware is dry before firing fire slowly below red heat (1100° F) where many changes occur in the clay and other materials provide plenty of air below red heat for oxidation and burn out organics and carbon do not to force cool the kiln while it shows red heat.

Making foodware safe   When making ware to contain food and beverages, it is very important to be sure it is foodsafe. Some of important considerations for mugs, serving pieces and dinnerware include: body composition design of the ware glaze selection decoration firing to maturity government regulations What Type of Ware? The design of some pieces of ware have inherent problems which make them unsuitable to contain food and beverages. Design-related cracks, rough areas, crevices and nooks and crannies are difficult to clean and might trap bacteria. They can also be difficult to thoroughly glaze. Pitchers with hollow handles can have the same problems. Ware also needs to be serviceable that is, it should be strong so it won't fail or break during service. Making Smart Glaze Choices While glazes are extremely durable, most are not completely insoluable. If attacked by acids in foods such as orange juice, vinegar and tomatoes, small amounts of the glaze may dissolve and pose a health hazard. Acid resistant glazes have passed rigorous tests and are labeled as foodsafe. These should be selected for glazing food ware. Lead-free glazes may not be acid-resistant and should not be used unless labeled as foodsafe. Homemade, altered, crackle, matte or specialty glazes also should be avoided for surfaces of containers that will contact food and beverages. How to Decorate When glazing, be sure to completely glaze the ware to ensure the entire body is sealed. Properly bisqued porcelain may be dry footed, but only if the porcelain has been fired to vitrification. Label the ware as foodsafe for future users. China paints, decals and rim designs are a popular way to decorate plates and mugs, but may not be safe for food surfaces. Specific regulations exist for the location of rim decorations which must be followed. Decals should be used on the outside of a piece where they will not be in contact with food or beverages. Use china paints on decorative items only. Safe Firing Proper glaze firing and the bisque firing are very important to insure ware is foodsafe. If the bisque is underfired, it may create problems with glaze and body fit that result in crazing of the glaze, or glaze surface defects such as pinholes. These would not be acceptable for ware used to contain food and beverages. If the glaze is not properly matured, it will not meet the foodsafe standards under which it was tested and may craze while in service. Using pyrometric witness cones on the kiln shelf is the only way to insure that a proper firing has occurred. For foodsafe ware, many prefer to fire their bisque to an 03 witness cone just to be sure it is fully mature. Read and follow the manufacturer's instructions for glazes for the best and safest results. Regulations There are several very specific regulations for ware which will contain or contact food and beverages. California has the most stringent rules for dinnerware and new standards have been set by the FDA for rim decorations. These rules are available from state and federal agencies. If you are selling your dinnerware you may be subject to additional regulation. How to Test for Lead Release Several easy to use products are available on the market to test for lead release. These are primarily quantitative tests - that is, they tell you yes or no if the surface has lead above a certain level. The most commonly used kit is a thick cotton swab which turns pink if lead levels are exceeded. This test does not harm ware so if it tests too high in lead, the piece can still be used as decoration. These tests are a simple, economical way to feel confident that your ware is safe. Want to learn more? Read more about Making Foodware Safe in the Orton Firing Line and Technical Tips publications. Each issue is packed full of articles to help you learn more about firing. Members of the Orton Firing Institute receive these publications at no charge. Single copies are available to non-members at a per issue rate. Orton's 80 minute video, Key Principles of Successful Firing, is also an excellent resource on firing.

 

 

Happy firing!

 

 

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