Modern slip clays are not just watered-down clays as might be used for decoration perhaps with colorants added, as an underglaze for example. Rather, they are a precise formulation of water, clay and a deflocculant which acts to reduce the amount of water required to maintain the liquid, pourable state and to keep the clay particles suspended in the water and the mix homogeneous. Less water results in far less shrinkage, greater strength, much faster set-up times and less wetting and thus drying time for the plaster molds.
Plaster (No. 1 Pottery Plaster specifically) is the preferred mold material as it provides the best combination of detail retention, strength and absorbability. The key in slipcasting of clay is the removal of the water from the slip once it is in the mold.
A deflocculant can be thought of as preventing the clay particles from "flocking" together (like sheep!) or getting lumpy. Technically, it is an electrolyte, which is an alkali, that changes the charge on the molecules or clay particles, causing them to repel each other like magnets which in turn makes the mixture more fluid (thus less water). The deflocculants used with clay follow:
SODIUM SILICATE ‘N’ The most economical and commonly used deflocculant for clay. Over many castings, it can accumulate on and seal the pores of the plaster mold rendering it useless. (Note: The ‘N’ designates the strength of the solution and all formulae mean ‘N’ unless specified otherwise).
SODA ASH Traditionally used as a secondary deflocculant with Sodium Silicate. It also makes the casting more elastic which may be desirable with elaborate forms that need to stretch in the mold prior to release.
DARVAN #7 Originally formulated as a glaze deflocculant, it is often used to "fine tune" a slip using the above two deflocculants or even in place of them. It doesn’t accumulate on the mold as readily and small differences in the amount used has less effect. Often preferred with porcelains and high iron content clays.
DARVAN #811 A newer replacement for Darvan #7 specifically formulated for slip clays but not yet in widespread usage.
Many variables are involved when mixing slip clays for casting including the acidity and hardness of the water. Since water is different in different areas and often changes during the year, any formula can only be a general guideline. The most important factor is to measureaccurately and to keep records including dates so the variables from batch to batch can be minimized and problem diagnosis thus enhanced.
The Basic Formula for low fire white slip and most other slips (often excluding porcelains and high iron content clays due to the different particle sizes in them) is:
100 pounds dry slip clay
.3% Sodium Silicate‘N’ (3 to 4 ounces by volume or 4.8 ounces by weight)
.05% Soda Ash if needed (1 ounce by volume or weight)
5 to 5.5 gallons water
Note: Sodium Silicate should never exceed .5%
In ALL cases, the finished slip should weigh 28 to 29 ounces per pint with a specificgravity of 1.75 on a hydrometer or 1.71 by weight.
SPECIFIC SLIP CLAYS
1. WHITE DIAMOND–Basic formula, 5 gal. water
2. 18-A STONEWARE-—Basic formula, 5.5 gal. water
3. TERRACOTTA–5 oz. (volume) or 6 oz. (weight) Sodium Silicate, 1 oz. Soda Ash, 7 1/2 gal. water
4. PORCELAINS–3 1/2 oz. (volume) or 4 oz. (weight) Darvan #7, 5 gal. water
5. SPECIAL MIX–Basic formula, 16 gal. water (150 lbs. clay)
Measure and add water first, then Soda Ash solution if used, then Sodium Silicate solution (leave a little until last to assure the proper viscosity), and then add the clay about a third at a time making certain the mixture is constantly agitated at least 10 minutes before adding the next ingredient.
Mix at least 5 hours, test for viscosity and thickness and add small incremental amounts of the remaining Sodium Silicate if needed.
You cannot mix too long.
Allow to sit overnight, remix at least 10 minutes and recheck viscosity and thickness before pouring.
A. 10 pounds of dry slip produces approximately 1 gallon of liquid slip
B. A good rule of thumb is that too little water is better than too much
C. Specific Gravity is the water to clay ratio and is measured with an hydrometer or can be calculated:
A=wt. of container filled with slip
B=wt. of empty container
C=wt. of container filled with water
D. Viscosity or fluidity is the ability to flow measured in time using a viscometer
E. Once you have determined the proper formula for your casting, it is recommended you only vary the sodium silicate
F. Slip should sit at least overnight if not for several days before being poured. It should then be thoroughly mixed and tested
G. It is usually easiest to mix the soda ash with an equal amount of warm water before adding it to the mix. The same applies to the sodium silicate (or Darvan)
H. Screening the slip through a 60 to 100 mesh sieve prior to pouring is recommended to remove any contamination, particularly if scraps are being recycled
I. Slip may be colored using oxides or stains but it is difficult to keep the colorants in suspension and prevent streaking, especially with oxides which are larger particle sizes. The deflocculant may have to be increased, especially if 5 to10% colorant is added for a deep color.
GENERAL PROBLEM DIAGNOSIS
A. OVERLY THICK SLIP (low viscosity) 1 and 2 below will also decrease the casting rate
1. Add sodium silicate in small increments (most common solution)
2. Add more water (also undesirably reduces specific gravity and can result in poor castings)
3. May be due to too much sodium silicate as evidenced by slip moving easily around sides of container as it is mixing. Add small amounts of epsom salt (magnesium Sulfate).
4. Add small amounts of barium carbonate (Caution:TOXIC) dissolved in warm water in 1 ounce increments if mix is high in sulfates as evidenced by scumming around sides of mix.
B. OVERLY THIN SLIP these will all also increase the casting rate
1. Add scraps from previous casting or more clay (and talc if low fire white) if too much water
2.Add Epsom salt if too much sodium silicate (see A-3 above)
3. Increase clay at expense of talc if low fire white
C. HARD SPOTS (Casting Spot, Flash Mark)
1. Decrease sodium silicate if mix is not already too thin
2. If on more than one casting and in same place, probably a hard spot on mold caused by multiple pourings hitting same place on mold and sealing the pores. Roughen spot on mold with fine abrasive.
1. Be certain mixer is not introducing air into the mix. Keep mixer at an angle and in the mix at all times.
2. Allow slip to stand overnight
3. Solidly tap the slip container to knock air bubbles to the top
NEARLY ALL OTHER PROBLEMS FROM BAD MOLD RELEASE TO CRACKING CAN BE TRACED BACK TO AN IMPROPERLY DEFLOCCULATED MIX. MEASURE THE VISCOSITY!