I'd been searching for months for lessons on how to make a plaster mold to slip cast clay. I took one a few years ago, and nothing stuck, not even the notes and handouts to this day which I still can't find. Also the product of the class was a plaster mold of a patty pan squash. And so when I found this workshop at Clay by the Bay, I emailed the instructor to inquire if we could cast an object of our choice because I don't want a mold of a vegetable or a rubber duckie, but something much more functional or for my own artistic purposes. And my instructor, Alex Simon was and is such a hoot. Literally, that's her below, blowing her trumpet to announce valuable information that we must not forget. Oh and did I mention that she loves unicorns, her Jewish heritage and sparkles?
I loved this workshop. I usually have to do something again and again, or at least 4 or 5 times, in order to remember the process to repeat it on my own. I feel a little more confident, enough to try and make my own plaster molds by maybe even making my own cottle boards. Day 1 was a lot of explanation and demo of a silicon pickle (I told Zan the pickles looked like vibrators) for her pickle pipes while I pondered which of the objects that I had brought would I cast. 
I winnowed down my objects to simpler shapes and these were my top choices of what to cast. I put away my lovely vintage tea cup and saucer and decided that would be a later project. If the object you choose to cast is irregular like a pickle, prop it on some lumps of clay for stability and measure and look at its dimensions to see what kind of orientation to make its cast and to draw the half way or seam line for its two halves. I opted for the plastic tea bowl I bought a couple days before at Daiso and which (after I filled its bottom) had only one undercut or in terms of plaster mold making--any protrusion, hole, cavity or recessed area not perpendicular to a mold's parting line. Also I wanted something functional to cast, something with a surface on which to etch or draw my own designs. I filled the bottom cavity of the cup with plasticine clay (containing enough oil so as not to be so stuck to another object) and because of the volume, filled the main cavity of the tea cup with Bmix instead of plasticine. I also cut a round of clay to mount over the tea bowl as a pour spout for my plaster mold. Next I drew a line with a sharpie down the exact middle of its two halves.
She also mentioned that she loves kitschy objects, and one of her rainbow-themed menorahs is going into the National Museum of American Jewish History in NYC. But check out this over-the-top vase. The turtles and frog are giving me ideas about how to make my plastic whales and dolphins into chess pieces for a sea-themed chess set.
I really liked this blue jean vase. I had to walk around the wheel to look at the front pockets of this vase.
Day 1 was so frustrating. I spent the first day claying up and then subtracting too much clay as I was trying to clean it up because I wasn't understanding how to orient it inside the cottle boards and imagining the pouring of the plaster over it to create the cast, which was okay as Alex kept reassuring all of us that this is not an intuitive process. I still went home, deflated and depressed and incredibly fatigued.
Before "claying up" my object or embedding it into clay in which to "finesse" into 90 degree angles at parting lines before surrounding the prototype with cottle boards, I admired Alex's ceramics. I mentioned her love of unicorns and sparkly things, but she's also very proud of her sexuality and was in full-mode PRIDE for this month of June celebrations.
My prototype at the end of the first day looked like shit. That night, I brooded over what I should have done differently and even pondered not coming back the next day, but then bucked up, determined that I would rebound the next day and just clay up more in order to fix and finish my tea bowl mold.
The next day, I used my yellow rib more than the metal serrated rib, which I had used to subtract too much clay. Per Alex's suggestion to smooth and finesse around those perpendicular angles, I used the yellow rib. I added more clay around the tea bowl to make it more level. There was no time to lament that my pour spout wasn't perfectly half round and smooth. Onward, I kept eyeballing for levelness and compressing and smoothing until I said good enough.
I do want to make more molds of maybe more irregular objects in the future and so paid attention to Alex's pour spout for her pickle as well as her further advice for smoothing and finessing the mold by using her favorite tool (the serrated and square metal rib) to also make 90 degree angles around the claying up, so that the cottle boards fit more squarely and snug against all that clay. 
Alex further demonstrated this on Jess's rubber duckie.
She then cleaned up all the clay remnants on the prototype by using a wet paintbrush and then brushing oil soap ( 1 part Murphy, 3 parts water--just little splashes really) three times, letting each coat dry at least 15 minutes and tossing the soiled brushes and mixtures or saving for later coating the cottle boards.
After her demo, I went ahead and used a cottle board around my own prototype as a guide for making my own slab smooth and square. 
See that gap above? That's what I did not want to see as evidence of my lack of squareness. And then it was time for a demo on the cottle boards.
She had used a trimming tool and a fancy melon-baller-looking tool specifically for making the keys in the mold (put at least 3 keys, but 4 if you've room for it and in an irregular pattern). The double-edges of the cottle boards go against the prototype. Clamps two at a time and diagonally from each other.
Then roll coils, thick and thin, for caulking seams on the cottle boards, where plaster might possibly leak. 
And then it was time to coil and caulk my own prototype.
I also walked around and looked at what kinds of things my classmates were making molds of. Basil didn't get the email about bringing objects to make a mold of, but he happened to have a leather-hard clay piece in the studio. Stephanie had wanted to originally cast a glass vase that had glass bumps all over, which Alex told her had too many undercuts, and so she cast an Irish coffee mug. 
Christine had brought a sweet potato. And I finally caulked my tea bowl.
Afterward, we were then instructed to take mallets and bang underneath the tables on which the prototypes sat.
Again the banging was to move more air bubbles within the molds to the top and not settle on the prototype. And then it was mine and Z and Christine's turn to mix the plaster and pour. 
After pouring my plaster and waiting for it to set up, I looked at Z's cottles made up of clay. And oh I don't remember if I took a pic of her removing her clay after the plaster was poured. Did she then put her hardened plaster into a cottle to pour more plaster for the other half? And had she just scored and slipped slabs to make that clay box around her clayed up prototype? 
I then watched Alex take off the clay that was up around Stephanie's prototype after she had rasped and rounded the edges of the plaster. 
Amazing. Stephanie's other half of the prototype was now exposed, and it looked cool.
The night before, I started looking to buy cottle boards for my own plaster mold making and looking at videos on how to make other types of cottle boards with flashing paper.
One tip from Alex was to buy C-clamps that are deeper than the ones we were using and for use with larger cottle boards.
Start measuring and figuring out to how high you're going to pour plaster, at least a couple inches above the highest point of your prototype.
Don't forget possible leaks through seams on the interior.
Plaster time! We had measured inside our cottle boards to calculate the volume (l x w x h) to determine how much plaster and water to use. We, Christine, Z, Tara, and I were a team and added all our volumes together for calculations. You start with a clean bucket of water and slowly pour in the plaster. Wait 3 minutes for it to slake.
And then mix with drill attached to a mixing blade for 2 minutes. Plunge your hand in and mix with an egg beater motion and feel the consistency of the plaster, which should be like heavy cream.
Alex explained as she swirled and jiggled the bucket that she was trying to get the air bubbles to come to the surface and not pour onto the prototype. She then poured the plaster slowly into the cottle boards containing the prototypes.
This is the time to clean off stray bits of plaster and clay, and make more 90 degree angles to the other half if necessary especially around the pour spout.
But really there was no time, and we were all in a hurry to get the second half of plaster poured, using the same calculation of how much plaster to mix for the first half. And the rest of the afternoon blurred into evening as I removed my clay, exposed the other half and then brushed it with mold soap and put cottle boards around it again and brushed more mold soap onto the other half of the prototype and the cottle boards. Alex had everyone put their finished molds on my table before the big reveal. I don't know if I had put enough mold soap over my newly exposed half of the prototype because it took ever so long and much later after Alex left and ever so much brute strength with a mallet from one of the studio assistants to pry my mold apart. 
In the meantime, Alex got dressed for a Pride event at Asia SF, and we all took a group photo before waving her goodbye.
Here are pics I took of Clay by the Bay and some of the pottery I admired. 
I really liked that dark brown mug with the checkerboard of orange and purple on the bisque and the coil pot.
I can't wait to learn to throw on the wheel I've borrowed and make pots as varied as the above.
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