Jeff Longtin

For those with a similar question I'll share this: soda ash is a very strong deflocculant. It is rarely used on its own and commonly paired with sodium silicate. ("N brand" as I know it.) From what I've read the sodium silicate works on the easy to defloccuate particles while the soda ash works on the harder to deflocculate particles. (As there aren't as many of those, in a casting body, you don't need as much.) Darvan 7 and 811 really came on the scene, for potters anyway, in the 80's after the publication of the book, "Moldmaking for Ceramics", by Donald Frith. (A wonderful book for those just starting to make molds.) Until the book came out most casters used the sodium silicate/soda ash combo to deflocculate clay. (Also adding barium carbonate to iron containing casting bodies, to reduce scumming.) The other reason soda ash is only used in small proportions is that it is very caustic. Caustic to humans and caustic to plaster molds.

Rereading your subsequent posting I'll offer this: if the "pointy details" are areas in the mold that have fine detail/sharp edges I would suggest you design these out of the mold. In other words, fill them in with clay, on the model, BEFORE you make the next mold. When making a mold, for myself or others, I try to create a mold that will result in consistent results. A mold that gradually wears away, after being exposed to casting slip, is not an effective mold. Instead I fill in the deep areas, before pouring the mold, and then instruct the customer to dig these out AFTER the piece has been cast. That way the piece comes out of the mold consistent with itself and its just a subjective decision how much "detail" is added to the finished pieces. I also contend that this detail usually is not noticed by the customer. I rarely hear people complain about sculptures not having enough detail. They do, however, notice/complain about inconsistent results. Regarding filling air bubbles after pouring plaster: its a nuanced process but I find it best to thin the plaster down to a "milk like" consistency. I then pour it into the air pocket. Quickly the water is absorbed and the plaster remains. The plaster does shrink, quite a bit, so you then need to apply another coat. I repeat the process until I have filled the hole. After the water is completely absorbed I then use a scraping tool to gently level off the plaster. If timed right I can get a nice smooth surface to the filled area.

A picture would help. Not sure if the "small pointy details" are plaster or clay? Too much mold release. Are you sponging off any excess mold release? Are you pouring while the release material is wet? Its best to make sure the surface being molded is dry. If its wet the mold release can absorb into the plaster surface. I use Murphy's and I sponge off the excess lather and then use a fine brush to remove any bubbles that remain.

Sometimes I combine mold parts to test out new design ideas. Sometimes the mold parts do not fit well together. I find filling the small holes/gaps with slip, FROM THE OUTSIDE, to be a simple remedy. Let the slip set up for a few minutes and you have a "sealed" mold. THEN fill the mold cavity with slip. You can also fill the gaps from the inside but I find it hard to get my hands inside smaller molds.

If you look at the website: ramprocess.com you will see the tabletop version. Looks like it goes for about 5K. An old model sits next to me, in case the big ones break down, and they are sweet little machines. The purge/vacuum aspect really expedites the pressing process. (You vacuum to suck the clay into the mold and purge to pop it out.) The 30 ton variety, in use, at work, are in use 6-7 hours a day. They allow for high production. I know a few studio potters who bought the 30 ton variety, thinking they'd make a fortune, but they're really not using them to their full potential. The tabletop variety is all most studio potters need. Repairing the things is a bit of a pain but the RAM folks are really great about walking me thru the process.

You have good air flow/purging so that's good. The design looks FAIRLY unobtrusive, so that's good. Thinking about it...the folks who use the presses, at the pottery shop, do allow their clay to stiffen. They are, however. using 30 ton presses. Can you use stiffer clay and pound it in using a hammer? (There's a video online showing such a process.) The RAM press also allows the user to throw the mold into vacuum. Does your device allow that? A drier surface could also help. Pure-Lube is described as a separating compound. (Like Murphy's Oil Soap.) It's typically used to separate plaster from plaster in the mold making process. If its working for Mark, great, but I would not recommend in this situation.

I work at a pottery shop where we have a few RAM press machines. I make several air-release molds each day. (I go thru a lot of Ceramical.) Your situation seems odd? If you're getting air bubbles, with no clay, it should be able to express the tile out. Can you post a picture of the mold and the tile form? Are you getting air over the entire surface? Could it be that when you made the mold the air gate moved or wasn't held it place securely? When you mixed the Ceramical did you follow the instructions? USG suggests water at 77 degrees. At one point I used really cold water and that affected the purging. Are you using old Ceramical? All bags are dated. What is the date of yours?

Bryan, For my braille cups I use a little 8 oz. plastic container. (The kind sour cream comes in.) I leave the lid on the container so the thin plastic retains its roundness. I work in a pottery shop where we make 700 pots a day. The folks who attach handles there like to use cork stoppers. (The 3" diameter variety.) Those work just as well. Sometimes I trim, with the plastic form in place, sometimes not. It just depends upon how things are drying. Because I attach the handle, after I remove the clay form from the mold, I try to remove it as soon as possible. I don't like to cut clay, against plaster, because it scratches the plaster and distorts the rim ever so much. When you cut the clay, outside of the mold, you can cut it "with the grain" and minimize the distortion. When you cut it against the plaster you're pushing it in one direction. (The one sided distortion reappears after firing.) I also leave the conical sprew on the piece until it comes out of the mold. I cut it, however, as soon as it starts to stiffen. (Soft leather hard, you might say.) After cutting the sprew I clean the rim with a sponge and then attach the handle. Once it becomes routine its not that slow and it improves the roundness considerably.

Being a slip caster myself I sometimes run into this. From my experience, 30+ years, its mostly a process problem. (A picture of your mold would help.) Placing a conical cup form into the casting, right after demolding, often reduces this problem. You can eyeball round but having an actual round cone form makes it much easier. Similar to a comment I made yesterday you can also minimize warping if you remove the piece from the mold and THEN cut the rim. My cup forms come out of the mold with the sprew still in place. I place the clay form on a banding wheel and cut the sprew with an exacto knife. I then place the cone form back into the cup and allow it to set up for awhile.

I was introduced to Seeley and Bell Porcelain slip back in the 90's. (When I made a few molds for doll artists.) I found both to be very white and very vitreous. I also noticed that both developed green mold fairly quickly. Recently I've returned to mixing my own and found that recipes that use English Grolleg Kaolin have the same qualities. (Whereas recipes with EPK or #6 Tile do not.) A recipe from claybucket.com is proving to work nicely. 10,000 gms water 80 gms Darvan 100 gms Bentonite 11000 English Grolleg Kaolin 4600 Nep Sye 4400 Flint