Day 8... Plaster Casting and The Barn

My "special pottery piece" is drying... slowing.. so it won' warp or crack. 

I plan to cast the piece in red clay, like the red clay my house is made of. Making a casting mold can be a long process. I usually make the original model 1/2" to 1" larger than the finished size of the piece. The original finished model is dried slowly and bisque fired.  This finished unglazed piece is given three or four coats of Murphy's Oil Soap.  Glaze would have filled in any details on the model. The soap soaks in and seals the surface without taking away any fine details. The soap also as a release. Plaster will not stick to the soaped model when I get to that stage. 



When casting with plaster, I must be sure te here are no traps or undercuts on the surface that will hinder the removal of the hard plaster later on. I use a marker to place divide lines where the seams will be on the model.  I roll out a piece of fine smooth clay and cut it into a presisely measured retangular shape an inch larger on all sides of the object. I lay the object on the center of this piece and start building my dams. I fill the base retangle of clay up to the lines I had marked on the model.  I use plexiglass frames to form a box around the model and dams at least 1" higher than the model. I check for any traps and fill those up with soft clay. I  seal the outer edges of the clay dams to the plexigass and also the verticle edges of the plexiglass walls. The plexiglass is firmly attached to the table with balls of clay on the outside and clips and heavy rubber bands around the outside walls.

Now I mix my plaster. You can use Plaster of Paris from the store, but it is cheaper and the plaster makes a better cast to buy #1 Pottery Plaster in 50# bags. I mix 1 cup of warm water to 2 cups of plaster. I mix it by hand, making sure lumps are dispersed.  Then I slowly fill in the cavity I had created above the model. Plaster sets up quickly and a few taps to the table will disperse any small bubbles. This will set in less than 1/2 hour. I gently remove the plexiglass, clay dams and finally the model itself.  I smooth the outer edges with a knife and check for any surface damage. If all is well, I make notches on two sides to be used as  "keys" so the pieces will fit into place like a puzzle. I replace the model to its new plaster form, and repeat the process for the other side of the mold.

Models are designed to have as few pieces as possible. A two piece mold is preferable, but my cow for the butter crock turned out to have 4 pieces because of the shape of the cows head.  Each plaster cast must have more Murphys oil soap applied to the outer mold surface only. Never put soap on the model image as this will not absorb the casting clay later on. 

When all the sides of the mold have been made, the original model is placed inside with all plaster pieces attached and a rubber band holds the hold mass together to dry. The mold must be completely dry before the casting can start.



Casting can be done with liquid clay. The trick is to have as little water as possible added to the clay so it will pour like cream. If there is too much water, the cast may never set up.  To make this possible, I put sodium silicate and soda ash into a bucket of dry clay scraps that have been soaked in water over night with just enough water to cover the scraps.  I have a large mixer to mix up 5 gallons quickly and smoothly.

In 1648, a man named Gauber made fluid silica by melting silica and potash. Plastic clay can vary from between 70% to 80% mass (weight)of clay  with the balance water. But this represents a volumetric ratio of about 50% to 70% water with the balance clay,  So the clay should fall apart and become totally fluid. It can be encouraged to do this with substances such as sodium silicate in a process called deflocculation. The sodium silicate coats each particle of clay and water can flow more freely in between. 





When the plaster mold is dry, the model is removed, rubber band put back around the mold to hold pieces together and casting clay or slip is poured into a top hole of the mold. The water in the clay is absorbed into the plaster and a thicker layer of clay will build up on the plaster walls. I keep an eye on the mold, till the thickness of the wall around the edge is about 1/4 inch thick.  Then the clay in the center of the mold which is still liquid is poured out . The mold is set aside over nite and the next day, the mold is opened. I trim the bottom or opening and dry the whole piece. Later, the seams will be scraped off with a knife and smoothed with a wet sponge. Finally, I am ready to fire the hollow piece. Each time a piece is fired, the particles of clay are melted together and the piece shrinks in size.

You can see it takes a long time before you can get a finished mold. Once the mold is made however, I can cast over 50 pieces before the mold starts to deteriorate. The salts in the clay will eventually cause pits in the plaster, and details will begin to fade. At that point, I will make a new mold by putting the original model back in place and recasting each piece in turn.

1959

It's nice to have a big old barn to cast and turn my pots in.  When grampa bought the property in 1927, the big old barn was larger, had verticle grey planks, a dirt floor and cow stantions. When my parents married and moved back here when I was 4, he and grampa did some renovations so he would have a place to repair cars.  Stantions and hay lofts removed, a rotting addition taken down, siding replaced with "decorative siding", two garage doors installed, cement poured for a floor. My little sister was a toddler and her foot prints are in the cement on the eastern wall.  By this time the cows and chickens were long gone, grampa was getting older, my dad needed a place to work and the barn was in disrepair. The hugh beams are still inside. Chestnut beams are attached with the wooden pegs.  My dad put in a furnace.  And now there is a piano wall.

I decided to learn to play a piano. My mom found someone to give me lessons and we bought a used player pianowith dozens of piano rolls. Grampa said he did not want a piano in the house because of the weak floors, (these floors are solid oak with trees and six and eight inch beams holding it up... I think he didn't want to hear me banging on that piano!). So we put the piano in the barn. My ambition to play did not last more than a few months, I am not musically inclined, but we used the piano rolls at the many cookouts we had in the back yard. Friends and relatives would come over and we played baseball, chrocet and horse shoes, ate lots of salads, burgers, hot dogs and drank beer, lemonade and Kool-aid. The garage doors were opened and we took turns playing the piano rolls and sing along to "The sidewalks of New York". It was great.

My parents and we three kids moved away for 10 years. The picnics were over, the barn without heat, and the piano sucummed to damp and cold. By the time I moved back in 1978, the piano was ruined. To make room for the pottery equipment, I took a hammer to it one day.  All but the thick back wall of the piano went in the garbage. The piano back with all its strings exposed is now set into the wall above my sisters footprints. Its a great conversation piece and kids love to pound on the strings.

Today

Now the barn has a different use. Adults and children stop by to make and buy pottery and wander through the barn marveling at the big beams and the piano wall.

Till next time... and more news on my special project item! Thanks for all those who are reading my ramblings and have sent me comments via email or on the phone, and all who have purchased pots for Mother's Day!

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