making your own deflocculated slip

What follows are my notes on how to make slip for slip casting. They are the result of reading a number of books, articles, pamphlets, blog entries, &c. Most of the technical observations originated elsewhere—I just codified them in a way that rendered them easier for me to understand and remember. If you are interested in doing serious slip casting, I can’t recommend Sasha Wardell’s Slipcasting (2nd edition) highly enough. Much of what follows is based upon her observations and processes. A little dry, perhaps, but is a powerful tool for learning this difficult technique. I also use Laguna’s advice on slip casting from a number of sources (links below).

Slip casting is certainly trickier than it might seem. But it’s worth it.

Outline

• Casting Slip Principles

• Recipes

• Warnings

• Process

• Testing & Slip Adjustment

Casting Slip Principles

• It might seem that ordinary clay slip—clay dissolved in water—would be effective in slip casting. Water molecules have a partial negative electrical charge near the oxygen atom, and a partial positive negative charge near the hydrogen atoms. Because of this electrical disparity, molecules (especially ionic molecules) dissolve in water. For instance, water splits ordinary table salt (NaCl) apart into two ions, Na+ and Cl-, which results in a solution with four elements: water, a positively charged Na, a negatively charged Cl, and some unsplit salt molecules (NaCl).

• Water thus works well to dissolve/ break apart/ declumpify all sorts of things by encouraging molecules to break up and repel away from each other. Unclumped in water, ordinary clay slip is pretty fluid.

• There are a number of reasons, however, why ordinary slip is unsuited for slip-casting.

  • Although water encourages molecules to break apart and repel away from one another, they don’t in all cases—not even in the case of table salt. Sometimes molecules don’t split apart in the first place, and even when they do, with all of the positive and negative ions floating around in a solution, sometimes they recombine to form their original molecules (and even different ones). Significant clumping is inevitable, and this is problematic given the fact that clumping is one of the great inhibitors in making successful slip casts.

  • Deflocculation is more effective because it works in a very different manner. It reduces clumping by neutralizing all the charges on the molecules so that they are neither repelled by nor attracted to one another, essentially rendering them inert. The particles thus remain independent (unclumped) without requiring water molecules to keep them apart.

  • This allows for a much denser slip solution, or a solution that is packed with more clay particles and fewer water molecules. The miraculous thing about deflocculated slip is that less water does not decrease fluidity. Much denser deflocculated slips are not viscous, but are just as fluid as thin slip solutions.

  • Maintaining fluidity is key in slip casting for a number of reasons. The fluidity of deflocculated slips ensures that

    • the slip dries uniformly on all sides of the plaster mould;

    • the slip reaches all parts of the mold when filling;

    • the slip can be swirled in the mold and evacuated without viscous clinging to the sides; and

    • heavier clay particles do not settle toward the bottom of the slip bucket (or the mould, for that matter).

  • The increased density / decreased water content is the second advantage of deflocculated slips.

    • Decreased water in the slip requires less time to be absorbed by the plaster, and the plaster mould doesn’t saturate as quickly.

    • Increased density of clay particles allows for stronger clay walls in the object.

  • There is a sweet spot, a specific rate at which the plaster mould pulls moisture out of a deflocculated slip solution which results in strong ceramics. The goal is to identify the range in which that sweet spot is to be found, and produce a deflocculated slip which operates in that range. It will take some testing, but creating the right slip will save you time, money, and not a little bit of heart ache. This is one of those cases in which care taken in preparation will pay dividends.

Casting Slip recipes

Adapted from Laguna Clay’s directions. Note that any slip should be fine-tuned before casting with it. Laguna’s is a base recipe that is a solid starting point for adjustments in the next step. Wardell has specific recipes for various clay bodies on page 85 of Slipcasting (2nd edition) if you would prefer to start there. I have never used these recipes, though, as I prepare slips using a Raku clay body, for which she does not supply a recipe.

5 gallons:

• 50 lb of dry clay

• 2 ½ gallons of water (9.75 l)

  • This will not seem like enough water for fifty pounds of dry clay. You will be surprised.

• Barium carbonate ¼ oz (7.37g)

• soda ash ½ oz (11.62g)

• sodium silicate solution (1 ¼ liquid oz sodium silicate + 1 ¼ oz water) = 2 ½ oz.

  • 1 ¼ liquid oz. = 2 ½ tablespoons (or 2 tablespoons + 1 ½ teaspoons)

  • Note: must dilute with water; never add sodium silicate directly to slip.

2 ½ gallons:

• 25 lbs of dry clay

• 1 ¼ gallon of water (4.88 l)

• Barium carbonate ⅛ oz (3.69g)

• soda ash ¼ oz (5.81g)

• sodium silicate solution (⅝ liquid oz sodium silicate + ⅝ oz water) = 1 ¼ oz.

  • ⅝ liquid oz = 1 ¼ tablespoons (or 3 ¾ teaspoons)

  • Note: must dilute with water; never add sodium silicate directly to slip.

Explanations. Sodium silicate is the principle deflocculator. Soda ash dissolves lignite aids in deflocculation (careful: store in sealed container lest soda ash morph into baking soda, which will thicken the slip). Barium carbonate neutralizes sulfates in the solution.

Warnings

• Use a respirator—it gets dusty.

• Never add sodium silicate directly to a slip solution, or all at once. Only work with sodium silicate that has been added to an equal amount of water by weight, and add the silicate solution in thirds as described below.

Process

Equipment:

• Gram scale

• Liquid ounce measuring cup (for the sodium silicate)

Setup:

• Measure the water into the slip bucket. there needs to be enough room to mix the slip—ideally, you should have a container that is at least ⅓ larger than the volume of solution you are producing.

• Mix the sodium silicate solution in a separate container.

• Add the barium carbonate and soda ash to a quart of warm water; add to the slip bucket.

• Mix for 5 minutes. It will settle in between stages—make sure to mix the solution before adding it to the slip.

Proceed by thirds:

• Slowly add ⅓ of the clay to the mixing water.

• Slowly add ⅓ of the sodium silicate solution to the slip.

  • Mix for ten minutes.

• Slowly add ⅓ of the clay to the slip solution.

• Slowly add ⅓ of the sodium silicate solution to the slip.

  • Mix for ten minutes.

• Slowly add the final ⅓ of the clay to the slip solution.

• Slowly add the final ⅓ of the sodium silicate solution to the slip.

  • Mix for three hours or until all the lumps are blended away.

Testing and Slip Adjustment

Having mixed your base deflocculated slip solution, you must now test it. Unfortunately, different water sources have different relative hardnesses; different altitudes result in different mixture densities; and different clay bodies, composed of different compounds, result in different densities. These variables can affect the fluidity and density of your slip. Testing the slip will let you know exactly how to adjust it to make your slip—composed of area’s water, and that particular clay body—especially effective. If you keep good notes, you will be able to reproduce the slip without testing it every time, but be aware that temperature and humidity affects the character of the slip, as well.

Waiting Period

Your slip must settle for 24 hours before you test it for viscosity. It makes best sense to wait 24 hours to perform both tests.

Equipment

The variables that you are adjusting for are viscosity and specific gravity. For accurate measurements, you will need the following equipment:

• a gram scale

• viscometer (to measure specific gravity)

Testing Process

Also adapted from Laguna’s directions.

Specific gravity test

• Weigh the viscometer (without the stopper). This is your empty weight.

• Fill the viscometer with slip to 500ml.

• Subtract the empty weight from the filled weight. This is the slip weight.

• Divide the gram weight by 500. This is the specific gravity of the solution.

  • Note: the specific gravity of water is 1. Because you are adding an extraordinary amount of clay, which has more mass, to water, the number will be much higher.

• Most casting slips work best between 1.72 and 1.80 specific gravity.

Viscosity test

• Fill the viscometer to 500ml and insert the stopper; place your finger over the hole and turn the flask upside down.

• Release your finger from the hole and time how long the slip flows before there is one second between drips. Laguna: “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.”

Alteration of the Slip

Laguna’s advice on how to alter a slip. Measure for specific gravity first.

Specific gravity:

• Too high: add water. Mix for 20 minutes.

• Too low: add clay. Mix for the full time (3 or 4 hours)

Viscosity

• Too high (more than 130 seconds/ 500ml): add deflocculant. Mix for 20 minutes. Never add water, as this changes the specific gravity.

• (It would be highly unusual for a slip with an appropriate specific gravity to have a flow rate of less than 100 seconds/ 500ml.)

Casting

Equipment: 60-mesh sieve

Make your slip at least 24 hours before your first casting. If you are using it in conjunction with a new plaster mould, be aware that it is customary to discard the first cast, as it usually picks of plaster bits, oil residue, &c., from the mould-making process