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Seeking any and all advice on making depression molds.

I need to make several one-shot molds varying in size, the largest roughly 10" x 20" by one to two inches deep. I'll be casting clear cullet or patties, probably Bullseye, but would welcome recommendations. (Have been perusing the archives, but specific tips for depression molds would be great.)

I've done this before but it's been many, many years. I believe I used 50/50 plaster/silica over a (wet) terra cotta form. I laid a piece of chicken wire in the wet plaster/silica for strength, though I wonder whether it made a difference. I got very little cracking, though for the work I'm doing, hairline cracking is not a problem. I don't recall my drying or firing schedule.

So, several questions:

I'd like to sculpt my positive in something firmer than terra cotta. I've used various grades of plastilene, but if I recall correctly, it didn't come out of the plaster as cleanly as the terra cotta. Thoughts?

What's the state of the art regarding plaster/silica mold recipes? I've read references to other materials such as Castalot which I'd be willing to try, but I haven't got the time for a lot of experimentation.

Any other advice, tips, cautions, etc. would be a big help: drying times, firing schedules, reinforcement, sources of info, etc.

Paul Housberg wrote: clip
I've used various grades of plastilene, but if I recall correctly, it didn't come out of the plaster as cleanly as the terra cotta. Thoughts?
clip

I've sculpted relief stuff on glass sheet with plasticene. THen I smoothed it with a hair dryer to get it glossy. THen cast it on the glass plate using 50/50 mix Silica flour/plaster ( I forget the water %). The molds came off quite cleanly. Held up OK to 1385F firings.

Paul

If you can get rubber positives they are the easiest way to make plaster negatives. Smooth-on makes dozens of rubber formulations. I like the one that has built in mold release. You could make a clay positive, a pure plaster negative and a rubber positive. Make it as thick as you want the mold to be. With no undercuts, you can simply pull up the rubber and the mold is ready to dry. Solid rubber is easier to work with than a thin coating of rubber with a plaster mother.

My take on plaster is to make a plaster/silica face coat, and back it up with a stiff mix including plaster , silica, grog, and chopped fibers. I use #2 casting plaster from the drywall supply place. I'm told that pottery plaster is not much different.

Paul:
Another technique you might want to consider is casting bllets in a box mold, creating a reverse felief. You would make a positive relief element in plaster and cast over it in a box. The method was taught and demonstratd by Ted Sawyer from Bullseye, and they now have a new TipSheet (#5) on the method. You can download the tipsheet from their site. The pieces done in our class using BE casting billets came out amazingly clear and needed little if any cold working. A key element is the use of a refractory wall mold made by screwing pieces of Skamol compressed vermiculite together. The mold is re-usable. A really interesting technique.
Nikki

Nikki O'Neill wrote:Paul:
Another technique you might want to consider is casting bllets in a box mold, creating a reverse felief. You would make a positive relief element in plaster and cast over it in a box. The method was taught and demonstratd by Ted Sawyer from Bullseye, and they now have a new TipSheet (#5) on the method. You can download the tipsheet from their site. The pieces done in our class using BE casting billets came out amazingly clear and needed little if any cold working. A key element is the use of a refractory wall mold made by screwing pieces of Skamol compressed vermiculite together. The mold is re-usable. A really interesting technique.

Bullseye is doing a demo on this at the upcoming Warm Glass Weekend conference.

Brad Walker wrote:

Nikki O'Neill wrote:Paul:
Another technique you might want to consider is casting bllets in a box mold, creating a reverse felief. You would make a positive relief element in plaster and cast over it in a box. The method was taught and demonstratd by Ted Sawyer from Bullseye, and they now have a new TipSheet (#5) on the method. You can download the tipsheet from their site. The pieces done in our class using BE casting billets came out amazingly clear and needed little if any cold working. A key element is the use of a refractory wall mold made by screwing pieces of Skamol compressed vermiculite together. The mold is re-usable. A really interesting technique.

Bullseye is doing a demo on this at the upcoming Warm Glass Weekend conference.

They are also going to show this technique at our next Portland chapter meeting of the Oregon Glass Guild.

Jim

Negative relief casting is easy and interesting.
The front surface is clear, it is the air surface
in the mold. The figure is in the rear surface and
has the (un)smoothness of the mold surface.

One interesting observation was that when I
casted a _positive_ relief of rounded female form.
It seems positive from both sides,
that is, seen from air and mod side.

The same with a sailboat behaved differently.
A sailboat (with sharp lines and large curved areas)
looks fine (in negative relief) fron the air side,
but looks negative -like the hull carved in water-
when seen from the mold side.

-lauri

jim simmons wrote:
They are also going to show this technique at our next Portland chapter meeting of the Oregon Glass Guild.

Jim

PDX chapter meeting: last Wednesday of September.
BB