Thursday, September 9, 2010

Slip Casting Step 1 - Mold Making

Having an interest with Slip Casting, we went to our friend Sherry's clay studio to experiment with slip casting. Sherry is casting a clay Russian doll and we were casting a (real) chicken wing!

The first step of slip casting is making the plaster molds. Both being first timers, we've only had extensive google and youtube research and an old casting book to blindly guide us, luckily we had an experienced potter at the studio who taught us tips and tricks as we went along the steps. We used clear plastic for the walls of our mold so the process can be easily monitored and recorded.

+ step 1: We made a 4 wall mold that will fit what you want to slip cast, make sure the mold is built in the right orientation so it's easy to place the chicken wings in, and at the orientation that allows the mold to have no undercuts.

Lining the bottom of the mold with clay

+ step 2: Because we want all sides of the chicken wing, we're making a 2 part mold. Fill up the bottom half of the mold with clay. place the item you want to slip cast into the clay, push it all the way down to the widest point and fill in all the holes so there wouldn't be any undercuts. Brush a releasing agent (we used soap water!) over the exposed surface so the clay, chicken wing, and plaster will separate easily. The last step of this before you make your plaster mix is to use the end of a pencil to poke locater holes in the flatter areas of the mold. This allows the 2 pieces of plaster mold to join together easily when casting.

Filled up the mold half way with clay

+ step 3: Mixing the plaster is a science in itself. we made the mistake of stirring the plaster as it was being added in. The correct way to make plaster is to measure out the water in one bucket, weigh out the plaster in another, and sprinkle thin layers of plaster into the water and watch the water naturally absorb the plaster. Once all the plaster is poured in and absorbed, put your hand in and mix it while crushing all the remaining clumps of plaster. There is a specific ratio of water to plaster to follow, but the consistency of wet plaster should feel like pancake mix!

Mixing plastic to the consistency of pancake mix

+ step4: to get the details of the skin and bones on the chicken wing, the best way is the flick your hand and sprinkle the wet plaster onto your mold. This creates a thin layer that will eliminate any large bubbles and retain the highest detail on your model. Once that is done, pour the plaster slowly from the side of the mold (not on the chicken wing), and vibrate to get all the air bubbles out.

Sprinkling wet plaster gently onto the mold

Pouring plaster into the corner of the mold... slowly

+ step 5: wait 20 - 30 min until the plaster is dry, flip the mold over and remove all the clay (make sure to keep the chicken wing (item) untouched). Apply releasing agent over the plaster and repeat steps 3 and 4 to create the second part of the mold.

Removing the clay

Part 1 of the two part mold

+ step 6: Once the second part of the mold is poured, make sure you wait 24 hours to let the mold set before the big reveal. We still have 5 hours and 23 minutes to wait ourselves...

Overall, we learned that...
1) Making plaster is an art and science, needs to be done fast and precise and that plaster hardening is a chemical reaction (which is why it heats up!).
2) Like concrete, do not pour plaster down the sink!
3) Raw chicken wing sitting in a plaster mold will not smell great...

- Jess
Special thanks to Rachelle and Sherry.