Making good slip is the first and most vital step in a successful slip casting operation. Although there are those who will tell you they’ve been using the same dixie cup for the last ten years to measure sodium silicate, and using a nail on a wooden barrel as a water marker, this approach leaves far too much room for error. Accurate measurements of weight and specific gravity are important. It doesn’t take a ceramic engineer to make a good quality casting slip. Slip mixing is a chemical process, which is affected by even the slightest changes in temperature, humidity and the chemicals added to your water supplies. Changes in conditions such as weather and water can affect the mixing formula, but, if you adhere to the following procedures, you will consistently make good slip. It should be noted that our suggestions are based upon average climatic and water conditions at our lab in Tacoma, WA. Adjustments may be necessary to fit your climate, water quality and general studio or plant conditions. For best results, let your casting slip sit for 24 hours and then fine-tune the viscosity before using it for casting.

1. Slip Mixer: Motor and blades which are capable of properly mixing your batch of slip.

2. Gram Scale: A scale for glaze chemistry is ideal, should be accurate to tenth of a gram. A postage scale is not accurate enough.

3. Viscometer: Developed to aid in controlling the quality of slip. It is used to weigh the slip, which will
show the ratio of clay to water in the slip, and to measure the viscosity of slip, which indicates the amount of slip that flows per second.

4. Graduated Cylinder or Extra-Large Syringe: A container measuring mLs or fluid ounces is used to precisely measure the volume of your deflocculant. If the container is large enough (60mLs +) this can also be used with your gram scale to measure the specific gravity of your slip by dividing the weight of the slip by the weight of the exact same volume of pure water.

5. Respirator: OSHA Approved style respirator is recommended

6. Sieve (60 mesh at least): A Talisman Rotary Sieve is an invaluable aid in screening your slip. You simply turn the handle and the slip is forced through the screen.


Sodium Silicate: Sodium silicate is a chemical used to deflocculate casting slip. NEVER ADD UNDILUTED SODIUM SILICATE TO THE SLIP BATCH. Too much sodium silicate causes over-deflocculation. With the batch formulas, we have given the ranges from the least to the greatest amounts of sodium silicate needed to deflocculate each batch. If you have had no experience with making slip at your location, measure the least amount of sodium silicate needed with your first one or two batches and “add as needed”. It is easier to do this than try to adjust a batch to which too much sodium silicate has been added. Keep careful records of each batch. This is important for future reference.
NEVER ADD PREMEASURED SODIUM SILICATE TO THE BATCH ALL AT ONE TIME, ADD ONLY SMALL AMOUNTS. Sodium silicate may be stored for extended periods in tightly closed plastic containers. (However, aluminum, galvanized iron, or zinc containers SHOULD NOT be used.)

Soda Ash: Soda ash works with sodium silicate to aid in the deflocculating. The correct combination of the two will give proper casting qualities. Soda ash works to dissolve lignites in clay. It works as a deflocculant, but if only soda ash is used as a deflocculant, 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 (it will thicken) instead of as a deflocculant.

Barium Carbonate: It is usually necessary to use Barium Carbonate in your slip batches to neutralize sulfates that are present in the clay, in the water, and/or leached from the molds used in casting. Barium Carbonate is toxic if ingested – wear gloves and be sure to wash your hands after handling and do not eat, drink or smoke while working with dry materials to avoid ingestion.

Dispersal Agents/DARVAN: Dispersal is an organic deflocculant which can be used with less fear of over-deflocculation. It is especially effective in hard water areas. Great to use towards the end of mixing to fine tune the specific gravity of the slip.

The hardness or softness of the water in your community will affect the results of your casting slip. Therefore, we will avoid recommending precise amounts of the ingredients in our formulas. Instead, a range of quantities will be given. We recommend that you begin with the minimum amounts of each ingredient and, if adjustments are needed, slowly and carefully add additional quantities.

Formula using 100 lbs. Clay Blend (Yield: 10 Gallons)
(Start with MINIMUM AMOUNTS Shown in formula)
• Clay Blend 100 lbs.
• Barium Carbonate 1/2 ounce (14.75 grams)
• Soda Ash 1 ounce (28.35 grams)
• Water 5 to 5 1/2 gallons (19.50 to 21.45 liters)
• Sodium Silicate* 2 1/2 to 3 1/2 fluid ounces
You must dilute this amount with a like amount of water. This will yield 5 to 7 ounces diluted Sodium Silicate.