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Ceramic Art Space is an authorized factory distributor for Laguna clay products
Starting in 2006 Laguna Clay no longer operates a retail store. If you need Laguna products
and you don't see them on our web site, please click on the contact us button and let us know.
We will try to get them for you.
Below you will find very useful information about the different Laguna products, applications/use charts.
Also below find how to make your own slip cast molds, MSDS sheets for clay products, how to make slip,
how to prepare dry glazes, firing ceramic bodies and much more.
Clays
When choosing a clay for a functional ceramic project, there are numerous factors to consider: color, plasticity, porosity, vitrification, firing temperature and glaze fit, just to name a few. The Clay Application Charts shown on this page are based on lab testing, input from our technical staff and feedback from our customers.Turn off pop up blocker then click on the chart to enlarge. |
Low Fire Clays
Characteristics:
Low fire clay tends to be either reddish or white in color and because of its porosity, glazing is required to create a waterproof surface. When fired, these clays are non-vitreous and have low shrinkage. Low fire dinnerware does not take extremely hot temperatures well, and repeated exposure to such temperatures can result in cracking and crazing. Laguna's low fire, high talc clays are used commercially, by hobbyists and in educational programs.
Traditional Applications:
- Low Fire Clays provide a good medium for small throwing, hand building and modeling projects. Recommended clays include: EM-207, EM-210, EM-330, EM-338, #10, #20. The following basic low fire clays contain sand to facilitate their use on somewhat larger forms: EM-212, EM-214, #10-G, #20-G.
- Sculpture Clays are designed for larger projects where varying thickness becomes a factor. Recommended Cone 06 clays: EM-215, EM-234, EM-337, #10-G, #26. Other low fire clays have specific uses which are usually described in their title, i.e. 06 #5 Pressing and 06 #8 Pressing.
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High Fire Stoneware Clays
Characteristics:
Cone 10 Earth-colored Stoneware Clays produce durable ware and most will react favorably to the manipulation of air, gas and smoke in reduction firing. These clays have good plasticity and will perform well in both wheel throwing and hand building methods.
White Stoneware Clays also have good plasticity and contain sand to increase their strength and workability, making them slightly open when fired. These clays have become very popular because of the clean canvas they provide for a myriad of glaze colors. Some iron specking can occur but for the most part, glaze colors are not affected.
Cone 5 Stoneware Clays are similar to Cone 10 clays in their workability and represent potential savings in lower firing costs. The lower firing temperature and oxidation atmosphere allow for the use of a large palette of ceramic stains and cause less warping of the ware.
Traditional Applications:
- Tableware may be made with stoneware clays with smooth to slightly coarse fired surface textures and low water absorption. Recommended clays include Cone 10: WC-370, WC-841, WC-379, WC-382, #900, #590, #510; and Cone 5: WC-877, WC-400, WC-401, WC-402, #65, #55, #50.
- Ovenware may be made from most stoneware clays provided the ware is well designed and the user is aware of potential thermal shock and how to avoid it. Cone 5 stonewares tend to be less susceptible to thermal shock but all ovenware made of stoneware should be heated and cooled gradually. Recommended clays include Cone 10: WC-376, WC-389, #750, #910; and Cone 5: WC-851, WC-877, #70, #60, #30.
- Wall Tile clay selection is completely dependent on one's individual design considerations. Recommended clays include Cone 10: WC-389, WC-376, #950, #310; Cone 5: WC-397, WC-394, #66, #45, #30.
- Floor Tile, on the other hand, should be made from a strong, durable clay to withstand wear and abuse. Commercial producers employ special production techniques to increase the durability and provide lower shrinkage. Recommended clays for floor tile include Cone 10: WC-371, WC-372, WC-381, WC 885, #901; Cone 5: WC-397, WC-365, #30, #60, #45.
Sculpture Clays contain significant amounts of sand and grog in various mesh sizes and are formulated for a low rate of shrinkage. These clays are designed to accommodate large hand-built, wheel thrown or sculpted projects. Recommended clays include WC-381, WC-371, WC-372, #901, #950 which are listed as Cone 10 but may be fired to Cone 5 or lower for sculpture work. WC-390, WC-392, #30, #66 and #75 are Cone 5 sculpture bodies, again often fired lower for sculpture.
Porcelains
Characteristics:
Porcelains are most commonly known for their whiteness and for having some degree of translucency. They are pure and vitreous which gives them their inherent glaze fit quality and unparalleled hardness and durability. Laguna offers a variety of prepared moist porcelain clays in firing temperatures ranging from Cone 10 to Cone 3.
Dave's Porcelain (WC-384) and #570 are outstanding Cone 10 throwing bodies and are also excellent for slab projects when a hard clay is desired. Cone 10 Windsor Porcelain (WC-879) is made with a highly plastic, imported kaolin, and translucent when construction is thin. A good Cone 5 porcelain is #15, and a pure white translucent Cone 6 porcelain is S-3126 (FROST).
Traditional Applications:
- Sanitaryware is usually made from porcelain or porcelain-like clays. Hardness and durability as well as flawless glaze fit are key factors in their function.
- Wind chimes create a nice sound when made of thin porcelain. If other less vitreous clays are used, consider glazing them to heighten their tone.
- Tableware, Tile, and Sculpture functions are greatly enhanced when made with porcelain. The smooth, white, hard qualities provide an ideal surface for glaze colors to achieve depth and clarity. Porcelain is waterproof and very durable, making its use for dinnerware especially popular.
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OvenCraft and Dry-Hard Clays
OvenCraft, OvenCraft II and Dry-Hard Clays are remarkable ceramic clays that can be thrown on a Potter's Wheel or hand modeled like any high quality sculpting clay, and all three will harden without having to be kiln fired. OvenCraft and OvenCraft II will harden after baking for an hour or less in your kitchen oven at 350 degrees, while Dry-Hard clays will actually reach an incredible hardness by naturally air drying for only a couple of days (each of these clays comes with detailed instructions). OvenCraft is available in a warm speckled brown, and OvenCraft II and Dry-Hard are available in buff, brown, white and terra cotta. These clays may be painted with any acrylic enamel, tempera or water color paint. They are more suited for decorative than functional ware, and are perfect for aspiring young artists.
Note: Forming methods, drying techniques and firing schedules will all effect your results with any clay. Always test an unfamiliar product before ordering it in quantity.
Glazes
The wide variety of lead-free, high fire glazes Laguna Clay offers represent formulas developed during the renaissance of high fire pottery over the past forty years. We continue to concentrate on developing lead-free formulas for our broad palette of glaze colors. Again, you will find detailed information describing the key characteristics of each glaze in our catalog as well as detailed instructions for brushing, dipping, or spraying.
Cone 10 Reduction
Cone 10 stoneware glazes are unique in that the firing plays such an important role in the process. Most of the Cone 10 glazes we offer are intended to be enhanced by reduction firing. Impurities and trace metals in the clay and glaze will react with carbon in the kiln atmosphere to create spotting. Reduction firing also causes glazes to attain a warm color and a smooth, durable surface quality.
Oxides react in various and sometimes spectacular ways depending on the glaze compositions. Each firing creates a certain unique quality in the ware. Recommended glazes include WC-527 Chun, V-18 #41 White, WC-556 Iron Red, WC-553 K9.
Cone 10 Oxidation
Most stoneware glazes can be fired in oxidation. Cone 10 oxidation firings come into play when more controlled, stable and reliable results are needed. Many commercial manufacturers employ high fire oxidation to produce durable ware with reliable results. Items like fine dinnerware, jewelry and sanitaryware are often fired in this range and atmosphere.
Laguna offers three base glazes in our stock Cone 10 dry glaze line formulated specifically for this application: SG-91 White Gloss, SG-82 Brilliant Black, and SG-93 Clear, formulated specifically for color development with or without the use of opacifiers. Also available are two clear base glazes with a Cone 5 to 10 firing range: WC-558 Jon's Clear and WC-520 #16 Transparent.
Cone 5 Glazes
Most of the lead-free Cone 5 dry glazes we offer are formulated to be fired in oxidation. Each has its own distinct surface quality, and reaction to colorants will vary from one to the next. Two glazes, SG-98 Light Blue and SG-99 Brown, are formulated to be used without additional color. SG-157 Clear and SG-138 White are both excellent base glazes for decorating with stains or color additions. Both glazes contain no zinc and allow the use of chrome tin stains.
Laguna's Moroccan Sand Cone 5 liquid glazes are for brushing application. This series features over 80 glazes in a wide variety of colors and surfaces. All of these glazes are lead-free and safe for food and beverage containers except for the four metallic glazes in the 900 series.
These liquid glazes in pints and gallons (most also available in dry form) provide an excellent vehicle for beginners to learn the ins and outs of glaze application. Those more adept in ceramics can utilize brush application techniques to create detailed and ornate designs.
Low Fire Glazes
Low fire talc bodies provide an excellent medium for ceramic art and a base for a large selection of commercially available textures, satins, satin mattes and vivid gloss colors. Laguna offers an extensive line of lead-free, low fire glazes and underglazes perfect for use with our low firing modeling clays such as EM- 207 and EM-210. Specify lead-free when you order.
The use of lead-free clear glazes over underglaze colors allows for precise color and design considerations. Laguna's EM-2001 is a spectacular crystal clear transparent, perfect for use over colorful underglazes. Also recommended are the lead-free EM-1000 Series Glazes and the EM-8000 Series Underglazes.
Some hints when using underglaze decorations:
- Bisque at Cone 05 - 04 and glaze fire to Cone 06. The higher bisque temperature will burn out material that might otherwise cause problems in the glaze firing.
- Leadless clear glazes are best when applied very thin. If the glaze surface feels dry, apply a thin coat of glaze and refire.
- White underglaze may be used to mute underglaze colors.
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| MSDS: Material Safety Data Sheets |
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Prepared Clays |
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Glazes |
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Raw Materials |
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Plaster & Mold Making |
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Refractories & Firing Supplies |
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How to... Make and adjust casting slip.
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Making good slip is the first and most vital step in a successful slip casting operation. Although there are those who will say they have been using the same Dixie Cup for the last ten years to measure Sodium Silicate, and a nail on a wooden barrel for a water marker, this approach leaves far too much room for error.
Accurate measurements of Viscosity and Specific Gravity are important. It doesn’t take a ceramic engineer to make a good quality casting slip, but slip mixing is a chemical process that is affected by even the slightest changes in temperature, humidity and the chlorine and chemicals that are added to water supplies. Changes in conditions such as weather and water can sometimes affect the mixing formula, but if you adhere to the following procedures, you will consistently make good slip.
It is common to experience significant variations in the amount of deflocculant required from one clay body to another. Low-fire, talc bodies, for instance, require considerably more deflocculant than Cone 5 porcelain bodies. It should be noted that our suggestions are based upon average climatic and water conditions in our laboratory in Southern California. Adjustments may be necessary to fit your climate, water and general studio or plant conditions. Also, for best results, make your slip 24 hours before using it for casting. |
| Equipment & Chemicals Necessary for Slipmaking |
- Slip Mixer
With a motor and blades which are capable of properly mixing your batch of slip.
- Gram Scale
With 500g attachment weight. A postage scale is not accurate enough.
- Viscometer
The principal quality control tool for slip. It is used to determine Specific Gravity, the ratio of clay to water in the slip, and to measure Viscosity, the property of flow.
- Liquid Ounce Measuring Cup
Sodium Silicate, for instance, must be measured by the liquid ounce, not by weighing it on a scale. 4 ounces of Sodium Silicate, measured by volume, weighs approximately 5 ounces on a scale.
- Respirator Mask
NIOSH approved.
- Sieve or a Shaker Screen
A Talisman Glaze Sieve (TM-363 in this catalog) is an invaluable aid in screening your slip. You simply turn the handle and the slip is forced through the screen. 60 mesh or finer recommended.
- Clock or Stopwatch
To measure seconds for the Viscosity test.
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- Sodium Silicate (N Brand)
Sodium Silicate is a chemical used to deflocculate casting slip. Too much Sodium Silicate causes overdeflocculation. (See the section on Over-Deflocculation.) Sodium Silicate may be stored for extended periods in tightly closed plastic containers. (Aluminum, galvanized iron or zinc containers SHOULD NOT be used.)
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NEVER ADD UNDILUTED SODIUM SILICATE TO THE SLIP BATCH. |
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NEVER ADD ENTIRE PORTION OF SODIUM SILICATE TO THE BATCH ALL AT ONE TIME. ADD ONLY IN SMALL AMOUNTS. |
- Soda Ash
Soda Ash works to dissolve lignite in clay. It works in combination with Sodium Silicate to aid in deflocculation. The correct combination of the two will give proper casting qualities, however, if only Soda Ash is used, your clay will become sticky. If Soda Ash is not stored in tightly closed containers, a chemical change occurs and it becomes Sodium Bicarbonate (Baking Soda). If this occurs it will then act as a flocculant (thickener) instead of as a deflocculant.
- Barium Carbonate (Toxic in raw form)
Barium Carbonate neutralizes (1) sulphates present in the clay or in the water and (2) sulphates leeched from molds used in casting.
- Liquid Ounce Measuring Cup
Sodium Silicate, for instance, must be measured by the liquid ounce, not by weighing it on a scale. 4 ounces of Sodium Silicate, measured by volume, weighs approximately 5 ounces on a scale.
- Dispersal (Darvan)
Dispersal is an organic deflocculant that can be used with less fear of over-deflocculation. It is especially effective in hard water areas. Darvan 7 is recommended for most applications, Darvan 811 is best for red slips.
DO NOT ALLOW DISPERSAL TO FREEZE.
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Mixing Formulas & Procedures
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You will find that different proportions of materials and chemicals will alter the performance of your slip, making it more or less desirable in your process. Therefore, we do not provide precise amounts of the ingredients in our formulas. Instead, a starting minimum amount is given. We recommend that you begin with these amounts and, if adjustments are needed, slowly and carefully add additional quantities.
A formula using 100 lbs. of dry clay blend will yield 10 gallons of slip. Remember, start with the minimum amounts shown below.
- Clay Blend
100 lbs.
- Barium Carbonate
1/2 ounce (14.75 gr.)
- Soda Ash
1 ounce (23.35 grams)
- Water
5 gallons (19.50 liters)
- Sodium Silicate
2 1/2 fluid ounces. You MUST dilute this amount with a like amount of water. This will yield 5 ounces of diluted Sodium Silicate.
SODIUM SILICATE SHOULD ALWAYS BE DILUTED 50/50 WITH WATER BEFORE BEING ADDED TO THE BATCH.
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- Before you begin mixing, put on a good quality respirator to protect you from inhaling dust.
- Measure water carefully and pour into the slip tank.
- Accurately measure the minimum amount of Sodium Silicate indicated for your batch and add a like amount of water (i.e. 5 oz. Sodium Silicate + 5 oz. water) in a measuring cup. Set aside. NEVER ADD SODIUM SILICATE FULL STRENGTH TO A BATCH OF SLIP.
- Accurately measure the amounts of Soda Ash and Barium Carbonate indicated for your size batch.
- Add Barium Carbonate and Soda Ash to one quart of warm water. (Warm water aids in mixing the chemicals.)
- Turn mixer on. Add diluted Barium Carbonate and Soda Ash (which you prepared in Step 5) to the water that is in the tank. Mix for 5 minutes.
- While the tank is still mixing, slowly add 1/3 of the clay indicated for your size batch to the water.
- Slowly add 1/3 of the Silicate solution (as mixed in Step 3) to the batch. NEVER ADD PREMEASURED SODIUM SILICATE TO THE BATCH ALL AT ONE TIME. ADD ONLY IN SMALL AMOUNTS.
- Mix approximately ten minutes, allowing the Sodium Silicate to thin the batch, then add 1/3 more of the clay. Allow clay to mix into the batch.
- Add 1/3 more of the Sodium Silicate solution. This will again thin the batch.
- Mix another 10 minutes, then add the final 1/3 of your clay to the mixer.
- Add the last 1/3 of the Sodium Silicate.
- Let the mixer run according to the following table, or until all lumps are blended away:
100 lbs. = 2 hours 1000 lbs. = 4 hours
300 lbs. = 3 hours 2000 lbs. = 5 hours
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Test Procedures
To determine if the slip is properly prepared, you need to measure two properties: Specific Gravity and Viscosity. You should measure and adjust Specific Gravity before you test and fine-tune the Viscosity. Specific Gravity can be tested after the mixing time indicated in the table above. Viscosity should be tested no sooner than 12-24 hours after the batch is mixed. This allows the clay particles time to hydrolyze, or become wet on all surfaces. If the viscosity is adjusted before hydrolyzation is complete, you are likely to over-deflocculate the slip. |
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Specific Gravity Test
After all lumps are blended away.
- Remove the stopper from your viscometer and weigh the dry, empty flask.
- Fill the flask with slip up to the 500ml mark.
- Weigh the filled flask on your gram scale.
- Subtract the weight of the flask from the total weight. The result will be the net weight of 500ml of slip.
- Divide the gram weight of the slip by 500 to establish the Specific Gravity. Example: A slip weight of 875 grams ÷ 500 gives a Specific Gravity of 1.75. Experience will teach you which weight is most appropriate for your casting conditions, and most casting slips work best between 1.72 and 1.80 Specific Gravity.
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Viscosity Test
12-24 hours after batch is mixed.
- Fill the flask to the 500ml mark and insert the stopper.
- Hold your finger over one hole in the stopper and turn the Viscometer upside down.
- Release your finger from the hole and time the flow of slip until you count one second between drops. Normally the slip will flow through in 100 to 130 seconds. A fast flow time (thin slip) can lead to a slower setting time and more brittle ware. A slow flow time (thick slip) will set up faster in your molds but render ware that is less dense.
- Tips:
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If the slip weight is normal, it is highly unusual for the flow rate to be less than 100 seconds per 500ml. |
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If the slip weight is normal and the flow time is more than 130 seconds, you may need a little more diluted Sodium Silicate or Dispersal. Never add water - it changes the Specific Gravity. |
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After testing, always wash your Viscometer with cold water and store at room temperature. |
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Adjusting the Batch
If your Specific Gravity is too low, add more clay to the batch. Then you need to wait while the addition mixes completely through the batch. With every clay addition, you should mix the batch for the fully recommended time (i.e. 3 hours for a 300 lb. batch).
If the Specific Gravity is too high, add water. If the Viscosity is too high, add deflocculant. You need to wait just twenty minutes for these additions to mix through the batch before re-testing.
If your Specific Gravity or Viscosity are not within the recommended limits, but your slip casts well, you should not make any adjustments! |
Over-deflocculation
Sodium silicate is the most common economical and powerful deflocculant. Unfortunately, it can quickly turn from your friend to your foe. Initial additions of sodium silicate serve to thin (deflocculate) the batch. However, there is a point after which sodium silicate starts to have the opposite effect, actually making the batch thicker (flocculating it). Compounding the difficulties is the fact that sodium silicate accelerates its effectiveness as it nears the point of over- deflocculation. For example, you might add one ounce to the batch three times, with little improvement in Viscosity, but the fourth addition will send the batch over the limit. That is why it is recommended to use small additions until you are familiar with slip making.
If you add sodium silicate and the slip gets thicker instead of thinner, it is over-deflocculated, and you have two options: you can try to recover the batch or throw it away. If your slip is badly over-deflocculated, it is very difficult to correct and will cost you more in time and frustration than the purchase of new slip. If the batch is slightly over deflocculated, you can bring it back with the addition of more dry material. Determining the correct amount will be trial and error, but with every clay addition, you should mix the batch for the fully recommended time (i.e. 3 hours for a 300 lb. batch).
If you believe your are close to over-deflocculation, it is time to switch from sodium silicate to dispersal. Dispersing agents such as Darvan can be added after you have added the maximum amounts of Sodium Silicate. You may add as much as 1 ounce of dispersal per 100 lbs. of dry clay without fear of over-deflocculation. |
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How to... Make a plaster mold.
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| The following useful guide to ceramic mold making is being reprinted with the permission of its author, Anthony D. Bulone, a renowned sculptor, designer, model maker and master mold maker for more than 35 years. Anthony of Solvang, as he is known, has excelled in most areas of the plastic and ceramic fields. His most notable accomplishments are the designing, sculpting and developing the model of the original Barbie Doll for Mattel 25 years ago and designing the first all plastic hobby kits for Revell, Inc. 30 years ago. |
| Materials Necessary: |
Tools Necessary: |
- Model
- Modeling Clay
Water base such as WED or #10.
- Plaster
USG No.1 Pottery
- Orange Shellac
For sealing clay and other porous objects.
- Parting Agent
Tincture of Mold Soap or Purelube.
- Baby Powder or Talc
For dusting prior to applying parting agent.
- Denatured Alcohol
For cleaning shellac brushes and thinning down shellac.
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- Wood/Wire Tool
- Wood Sculpturing Tool
- Metal Spatula or Knife
- Casting Boards (4)
- Work Board
- "C" Clamps (4)
- 3" Putty Knife
- Screwdriver
- Hammer
- Rubber Mallet
- Round File (coarse)
- Ruler or Yardstick
- Metal Spatula or Knife
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- Assorted Brushes
- Scale (0-60 lbs.)
- Metal Square
- Wooden Wedges
- Drill Motor, 1750-2400 rpm
(shaft with 3" rubber disc) or Large Enamel Spoon
- Scooper
- 5 Gallon Plastic Container
- 1 Gallon Plastic Jar
- Stainless Steel Scraper
- Rubber Bands (banding)
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Making a plaster mold
Procedure:
Models made of clay, wood or plaster must be coated with 2 or 3 layers of thinned down orange shellac.
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Step 1: First you must study the object from which you are to make the mold, to establish the Parting Line. Draw a line on the object with a dye-marking felt tip pen. |
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Step 2: Place the object on your Work Board so your parting line is somewhat parallel to your work surface. Proceed to Clay-Up with water base modeling clay around the object to your parting line, extend out for one inch from the widest points. |
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Step 3: When clay is all in place, smooth and leather hard, square off clay as shown. Using a soft brush, apply two thin coats of Orange Shellac over the object and the top surface of the clay parting line. After 15-20 minutes, apply talcum powder, dusting lightly. |
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Step 4: Apply Parting Agent with a soft brush covering the entire surface. Dry your brush and pick up all excess Parting Agent, leaving a very slick surface on the object and parting line clay. |
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Step 5: Prepare your casting boards, wiping each with Parting Agent on front surface and edges. Assemble as shown with "C" Clamps. With wooden tool, seal edges where clay parting line meets the insides of the board as shown. |
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Step 6: READY FOR CASTING. For the size of the object shown in these drawings and the apparent bulk of parting line around the model, we will use the following proportions: With your 1 gallon plastic jar, weigh out 1 1/2 lbs. of No.1 Pottery Plaster |
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Step 7: Let the plaster soak for 3-5 minutes, then mix with your Drill Motor Mixer. Mix for about 1 1/2 minutes, then pour over boxed pattern. Plaster should cover at least 1 inch over the highest point on the Model. |
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Step 8: After 20 minutes, take casting boards apart, scrape off top of plaster and bevel the edges slightly. Grasp opposite sides of the plaster/clay mold and gently twist to loosen clay from the work board as shown. Turn mold over and proceed to lift off clay from model and plaster half of the mold. |
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Step 9: If orange shellac transfers to plaster half, clean with alcohol. Clean all clay particles from the mold surface. Now is the time to carve Keys into the plaster parting line. You can make Keys several ways - round end of large spatula, a coin (nickel or quarter). |
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Step 10: Prepare your casting boards again, dust the model and parting line area with talcum, brush on parting agent as in Step 4. Assemble the boards as in Step 5 with "C" Clamps and repeat Steps 6 and 7 for casting the second half of the mold. Let the plaster set for 1/2 hour, remove the casting boards, scrape top of the mold, bevel the edge and corners. |
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Step 11: Now you are ready to open the mold. Scrape off any plaster that may have run down the side of the first half of the mold. Using a flat end screwdriver or a wooden wedge, insert it at the parting line; tap it gently with the hammer. As soon as the mold starts to part, turn the mold over and repeat the process. When the mold is loose, grasp each half and gently pry apart. |
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Step 12: The model will usually stay in half of the mold. At this point, how accurate you were with your parting line, what material your model was made of , and how hard or soft that material was, will dictate how easy or hard it will be to get the model out of the half plaster mold. |
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Step 13: If the model was made of clay, you can ease it out by using the screwdriver. But if you have to do any prying like a lever, place a flat piece of wood under your screwdriver so you won't chip the plaster mold. If your model is made of metal, glass or ceramic, you might have to use other means, such as air pressure or tapping all around the model with a rubber mallet. In some cases, I have chipped out the model thus destroying it. But keep in mind, that at this point the mold is the main object because a good, usable mold can reproduce 50 to 150 objects. |
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Step 14: With both halves now clean of any particles, we now determine where to carve in your Pour Hole. It can be in one half or in both halves as shown. Bevel the outer edge of the parting line on both halves and bevel all outside edges of the mold. This keeps that edge from chipping. |
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Step 15: At this time, check each half of the mold for any under-cuts that can be cut back. Let your mold dry out for 4 or 5 days depending on your weather conditions. NOW YOU HAVE A MOLD. |
Glossary of Terms:
- Clay-Up - Filling in around a model to develop the parting line.
- Leather hard - Clay dried to consistency of leather.
- Model - Object that is used to make a mold over.
- Parting Agent - A material used over an object that leaves a slick film.
- Parting Line - The line which divides your model.
- Work Board - Board on which the model is prepared for casting.
- Dry Brush - Wiping your brush so it will pick up excess material from the model or mold.
- Keys - Round or oval shape depressions with matching counterpart usually on flat section of parting line of mold.
- Pour Hole - Carved inlet or opening in mold to pour ceramic or porcelain slip into.
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Making your own Casting Boards
Casting Boards are used to form a box around a clay-up object before casting your plaster mix into it.
These drawings show two different ways to make corner angles for clamping the boards together.
Boards can be moved in or out from each other to make a small or large square or rectangle. Boards of varying sizes can be made. |
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| How to... Prepare dry glazes for application.
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When preparing a glaze for pouring, dipping or spraying application, the addition of Bentonite is usually required to help keep the glaze in suspension and increase the drying time. If the glaze is to be used for brushing application, C.M.C. Powder and Bentonite must be added. C.M.C. acts as a binder and allows the glaze to flow smoothly.
The following instructions are intended only as a general guideline:
- For brushing only-Mix C.M.C. Powder (22 grams) with a small amount of warm water (32 ozs.) and age for a day or two.
- Blend Bentonite into the dry glaze.
- Measure water into a clean plastic bucket of adequate size.
- Screen or sieve dry material slowly into the water while continuously mixing. Scrape sides and bottom of bucket often and mix thoroughly.
- For brushing glaze add C.M.C./water solution and mix thoroughly. (See No. 1 above)
- Check viscosity with a hydrometer. The hydrometer reading should be approximately 55 for dipping color glazes, 46-47 for dipping clear glazes, 60 for spraying and 60-65 for brushing.
- Glazes should be screened through the appropriate mesh to achieve the results desired. (Typically, 30-80 mesh sieves.)
- Glazes are usually best when they have been mixed and allowed to sit for a period of 24 hours before being remixed and then applied.
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Dry Glaze Mixing Chart
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| Dipping Glaze |
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1 lb. dry |
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10 lbs. dry |
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50 lbs. dry |
| Water |
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10.5 to 11.5 oz |
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105 to 115 oz |
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4 gal 13 oz to 4 gal 63 oz |
| Bentonite |
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0 to 2.3 g |
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0 to 22.6 g |
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0 to 114 g |
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| Spraying Glaze* |
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1 lb. dry |
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10 lbs. dry |
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50 lbs. dry |
| Water |
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7.5 to 8.5 oz |
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75 to 85 oz |
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2 gal 119 oz to 3 gal 41 oz |
| Bentonite |
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1.1 to 3.4 g |
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11 to 34 g |
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57 to 170 g |
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| Brushing Glaze |
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1 lb. dry |
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10 lbs. dry |
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50 lbs. dry |
| Water |
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6.5 to 7.5 oz |
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65 to 75 oz |
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2 gal 69 oz to 3 gal 9 oz |
| Bentonite |
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4.5 to 6.8 g |
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45 to 68 g |
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227 to 340 g |
| C.M.C. Powder |
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2.3 to 3.4 g |
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23 to 34 g |
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114 to 170 g |
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| * The use of a when spraying is highly recommended. |
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Cracking and Thermal Shock
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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.
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Cracking and Warping Caused by Drying and Casting
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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.
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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.
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ACRYLIC PAINTS / BISQUE STAINS
