Flat pieces bending...

Unless there is a design on them, you can sandwich them between some flat object like sheetrock or the like forcing them to dry flat. You just want to create an environment where the rate of drying is equal in all directions.. otherwise, covering them with plastic and letting them dry ever so slowly may be your only other option.

What are they sitting on when the are drying?

Diego

Slow drying and having something flat on them to make sure they cannot warp. The outside edges always dry first, this causes the cupping.

They dry part of the way in the mold, which is open on the top. Then I turn them out on a glass pane to make sure the other side has smooth edges.

What they are is 6 in wide, 1/2 in thick trivets. I have the shape in a open mold, I fill it with slip, and let it dry. MAybe I should pour off the excess? I was letting it set because I want mass in the piece, but if I poured it off it might dry more evenly.

yes pour the excess back then place a sheet of thin wood over the mould and invert to release.using glass will make the underside remain wet as moisture cannot escape whereas it will soak into the wood (or piece of dry wall). then place another piece on top so it will not warp when drying.

I am trying to make a very flat and smooth piece of mold plaster for this purpose. Will that work? I really want the open side to be VERY smooth (more of my accuracy problems), wood probably won't work unless I polish it.

I will see if drywall is available in small pieces.

in article lWXWb.32312$ snipped-for-privacy@twister.tampabay.rr.com, Noname at snipped-for-privacy@noname.com wrote on 2/12/04 9:57 PM:

Pouring off will cause slumping in the drying process. I do some similar work. home.earthlink.net/~martin_kenny

In my tile making projects I have experimented with drying the tiles on flat pieces of bisque. I've made the bisque dying slabs and dried them carefully under weight covered and very slowly. Its worth the effort because I find the bisque slab absorbs water at a very fast rate lessening the chance for warping and to be doubly sure I have put a slab on top. This can only be done in small batches of tiles at a time,but I've done a few kitchens this way and made money on the deal. You can make the surface of the bisque slab as smooth as you like by burnishing. Personally I hate plaster in my studio somehow sooner or later it shows up in the wall of a pot ruining it. Nancy Jane

v

Like Nancy Jane I will not have plaster in my workshop. The exception is what we in the UK call plasterboard, used to cover stud walls before they are coated with plaster. I use plaster Lath, strips of plasterboard used for ceilings. I tape over the cut ends with wide masking tape to prevent any crumbs of plaster escaping and use them for high speed drying. Tiles and the like dry underneath at the same speed as the top surface. Another material I use for drying slops is a Vermiculite based fire-resistant board known over here as *Vermiculux*. It is available 1 inch thick which is handy. When not used for drying it stands against the wall, thereby releasing bench space for other operations, much needed in a small workshop like mine!

Steve Bath UK

In article , Nancy Albin writes

Hmmm... so the advice I have found to make my molds out of plaster may come back to bite me?

NoName

I don't have much experience with pouring slip into molds. I don't know the systems you can set up to keep things from mixing together. My approach to pottery is basic and artistic and pretty free flowing so heaving slabs around when an idea hits can cause havoc as it has for me. Especially one time when my artist son was making a big project of torsos using plaster molds ( I would only let an offspring of mine commit such an offence in my studio) I still have nightmares about my anxiety over that one.

David Davis a well known potter in this area makes huge pressmolds of animals out of plaster, my son worked for him years ago and said that somehow Davis keeps the plaster out of his clay but he doesn't glaze his pieces. Guess it just comes down to personal taste,work style and what you can put up with Nancy Jane

v

I guess I should ask, what happens if you get plaster on your clay? Can you get it off?

Plaster "on" the clay is not a problem. Any damp cloth or sponge can remove it without too much difficulty.

The problem is with plaster "in" the clay, as can happen when plaster is worked in a studio. Some plaster chunks, pieces dust or bits invariably find their way into your clay supply.

When fired, the plaster produces "lime pops" which are little blowouts on the surface of the piece. I've seen them blow whole chunks out of a piece, destroying it, depending on the size of the plaster incorporated into the clay, as well as the relative strength of the clay body. If you can, use an alternative to plaster, such as doubled or tripled pieces of canvas, or even the "Hardiebacker" cement Board used to install tile over drywall. If you are planning on using plaster as a wedging surface, might I suggest that you cover it with canvas first, since your wedging surface is going to need to be cleaned occasionally. Drywall can also be used (some know it as "sheetrock" though that is a brand name) but drywall carries it's own problems, as it is mostly composed of paper covered gypsum. Gypsum can also cause problems in firing. Some people prefer to use a 1-2 inch slab of concrete, poured and smoothed, instead of any of the others mentioned above. Heavy (so is plaster) but it will last a lifetime.

Bear in mind that there are a number of different "grades" of plaster. Some are much more durable and "hard" than others. Depending on what your intended use is will determine what type of plaster to use, if any. Just remember, clay and plaster don't mix (well). Hope that helps, Wayne Seidl

Thanks, it does help.

(wishing I had time and money for a class...)

Just to put all Wayne's excellent advice into perspective, note that plaster wedging tables have been a standard practice for many potters for ages. Just don't use sharp tools to clean it, only a damp rag.

As far as I know, it's only plaster bits, not dust, that are the problem. Plaster is partially-hydrated calcium sulfate, made by heating gypsum a little to drive off some of the water. In firing it turns to calcium oxide, which is a flux that will enter into the clay/glaze at high temperatures. I don't recall how high of temperatures, but I think fairly high. But the calcium oxide is formed in bisque firing, and it is hygroscopic, so assuming that you don't later fire high enough to melt it, it will always be lurking in the piece to attract some humidity. When it does, it expands.... hence the lime pop.

But if the plaster is a fine dust, there isn't enough local expansion to cause a problem. Some folks claim that

30 mesh or smaller is enough to prevent problems, but I'd say that's not quite small enough... maybe 80 mesh would be safer.

Certainly if you have room to keep your plaster area separate from your clay area, it would be a good idea. But if not, just use reasonable caution. And note that slip-cast pieces are almost always made in plaster molds, so it can't be all that bad a problem!

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

Wayne, isn't plaster and Gypsum the same? It both translates into Gips in german. I use plaster as a wedging surface, or mostly as a drying surface for recycled clay and have never had any problems with it. (I must say, i am a rather neat potter, plaster pieces don't end up in the clay). ("Lime pops" are referred to as "Kalkspatz" in german, which translated means "lime sparrows". Don't ask me why).

Monika (Vienna, Austria)

-- Monika Schleidt snipped-for-privacy@schleidt.org

Monika: That would make sense, since sparrows "fly" and so do lime pops. :>) I was taught that spatz translated to dumpling as in "spatzle", another translation that works since a dumpling is not actually a "part" of the soup, but only cooked in it.

You are right in that Gypsum and plaster are almost the same. See Bob Masta's excellent post in this thread. I have learned a lot about a problem I hope never to have...

Best, Wayne seidl

Another problem item is blackboard chalk; I used to have a blackboard in my workshop to make notes on. One day my kids had a chalk fight while I was out of the room and several pieces of chalk ended up in the slops bin, from thence into the re-cycle bin, and ultimately into my production! when I started getting blow-outs in my bisque stock I investigated and soon arrived at the cause. The result was that more than 500Kgs of clay had to be ditched and most of my bisque, and I had to restart from scratch! It put my production back a month. Needless to say the blackboard went.

Steve Bath UK

In article , wayneinkeywest writes

in article nMrXb.233802$ snipped-for-privacy@twister.tampabay.rr.com, Noname at snipped-for-privacy@noname.com wrote on 2/14/04 10:10 AM:

don't worry about the plaster. be careful not to contaminate the clay - not hard to do. just don't throw the plaster down the drain unless you can afford a big plumbing bill.