Ceramic Handbuilding

I took a ceramic handbuilding class in college. I wasn't the best at it and have no desire to become a sculptor or throw on a wheel. However, I was inspired to make my own stone bakeware and earthenware pots. As luck would have it, I live in Oklahoma, the home of the famous Frankoma Pottery. John Frank was a ceramics professor at OU from 1927-1936. 

 

Oklahoma is known for its iron rich heavy clay soil. However, the Arbuckle Mountains is home to a light tan clay deposit called Ada clay. In 1933, Frank began using Ada clay. In 1938, he moved his pottery company to Sapulpa and hauled the Ada clay 150 miles to his studio. 

In 1953, he switched to red Sapulpa clay and the rest is history. 

Frankoma pottery is best known for its prairie green glaze over red Sapulpa clay that is still dug by hand.

Julie Bradley of Bradley's Pottery in Chandler, Oklahoma specializes in making her own clay from a hand dug pit. She calls it Route 66 Oklahoma red clay. Her beautiful wares showcase the natural color of the clay.

Another potter, Earline Strom, also hand digs local clay for her Red Earth shop in Stillwater. 

Jeri Redcorn of Colony, Oklahoma, is responsible for reviving Caddo pottery and pit firing. She discovered that the clay around the Red River is extremely workable. Her vase, "Intertwining Scrolls," was picked by President and Michelle Obama for display in the Oval Office of the White House.

Chase Kahwinhut Earles, is also a traditional Caddo potter who gets his clay from the Red River and pit fires it. He learned the art from Jeri Redcorn and teaches classes.

Mineral clay is created by the direct erosion of mountains. This is the type of clay that is mined commercially and suitable for kiln firing. Alluvial clay is created by rivers washing away the top soil exposing the clay. Alluvial clays have a higher concentration of iron oxides and a lower vitrification temperature. They also require a coarser temper to survive pit open firing.

This video shows how to process red clay dug from a backyard.

This video shows how to process alluvial clay from a river.

Tierra Negra (Black Earth) pottery from Columbia serves as cookware, bakeware and serverware. It is fired in an earthen kiln.

Clay Body Paper Gluing Slip Recipe

4 squares of 2-ply toilet paper

1-1/2 C. Distilled White Vinegar

2 C. Bone dry clay, crushed to pea size pieces

Place toilet paper into a pint size plastic container and pour vinegar over the paper. Make pulp with an immersion blender. Add clay and mix with a wooden stick into a paste. Allow to sit for 24 hours. It will get foamy as the vinegar breaks down the clay. Adding a drop of bleach will prevent souring. 

Works for joining soft pieces to soft bodies. First brush on slip to the body and piece and then, score them. Also works for filling cracks in soft bodies.

Clay Body Gluing Slip Recipe

1 gallon Distilled water
3 Tbsp. or 9.5g liquid sodium silicate
1-1/2 tsp. or 5g soda ash

Mix the water first. Potters refer to this as Magic Water.

1-1/2 C. Magic Water

2 C. Bone dry clay, crushed to pea size pieces

Pour the magic water into a pint size plastic container. Add clay and mix with a wooden stick into a paste. Allow to sit for 24 hours.

Works for joining soft pieces to greenware and filling cracks in bisqueware.

Clay Body Casting Slip Recipe

100 lbs. Clay powder

14.75g (1/2 oz) Barium Carbonate
23.35g (1 oz) Soda Ash
5 gallons Distilled Water
2-1/2 oz Sodium Silicate diluted in 2-1/2 oz distilled water

SODIUM SILICATE SHOULD ALWAYS BE DILUTED 50/50 WITH WATER BEFORE BEING ADDED TO THE BATCH. Clay bodies with high iron content will require less deflocculant than others. 

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.

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 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 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

Measuring Specific Gravity

You’ll need to measure the specific gravity of the slip to determine if the water to clay ratio is correct. First tare (zero out) the weight of a graduated measuring container on a scale then fill it with 100 ml of slip and see how much it weighs in grams. Divide that weight by the weight of an equivalent volume of water (100g). This moves the decimal point to the left by two places. Example: 183 g = 1.83 specific gravity.

In general, a specific gravity of 1.80 to 1.85 provides good balance of fluidity and strength for most casting purposes. Different clay bodies have different optimal specific gravities. Once you find a number that works, write it down and stick with it for that clay body. If you have a specific gravity that is higher than 1.85 and you want to bring it down, add a little water (only a few drops at a time), to your slip, mix and re-test. If the specific gravity is lower than 1.80, let the slip sit 24 hours, skim the water off the top, remix, and measure the specific gravity again.

Viscosity Test

12-24 hours after mixing

1. Fill the flask to the 500ml mark and insert the stopper.
2. Hold your finger over one hole in the stopper and turn the Viscometer upside down.
3. 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.
4. Tips:
   

   A.	If the slip weight is normal, it is highly unusual for the flow rate to be less than 100 seconds per 500ml.
   B.	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.
   C.	After testing, always wash your Viscometer with cold water and store at room temperature.

Use the drill to mix the deflocculant into the slip and use a ladle or pitcher to scoop some of the slip out to see how it pours. The consistency of cream is what you’re going for so be careful not to over-deflocculate the slip, which will cause slow uneven drying and soft spots in the finished piece. Just add a few drops at a time to achieve fluidity and remember a little goes a long way with deflocculants.