I recall reading somewhere that adding a small amount of dry kiln wash to dry plaster/silica before mixing the lot with water result in a very clean cast.
However, I can't find the original post or article - I may have imagined reading it
You probably read something I wrote, because it's something I do frequently and I mention it quite often in my blog. I sometimes add a dollop of kilnwash to the face coat of plaster/silica molds because it does seem to facilitate a clean release. Typically, I weigh out the mix as I normally would, leave it a bit scant, and sift enough kilnwash into the mound to take the total weight a bit over. Then I slake and power-mix as usual.
Given that most of my face coat mixes are actually dental stone/alumina with a little EPK, and kilnwash is typically rich in alumina and kaolin, it's probably coals to Newcastle. I think the only real advantage is that you use a commercial kilnwash such as Bullseye's (which is pink) or Primo (which is purply-blue), it'll tint the face coat so you can distinguish it from the rest of the layers.
More often, though, I'll do this with quick-and-dirty single-layer molds, especially if I'm just doing a single pour, as in a box mold. They're usually smaller molds, I'm not getting all fancy with the special alumina face coat, it's just plaster-silica, cheapest and fastest I can get. I dump kilnwash in with the dry mix, let it slake, power-mix it, and it definitely improves the surface quality of the final mold.
I sometimes use kilnwash on molds I've made, too. I mix up a looser-than-normal batch of kilnwash, pour it into a fresh mold until it reaches the top, then immediately pour it out. As long as the mold is still damp, I'll get a thin coating of kilnwash that doesn't hide detail but delivers a really nice, satiny finish to the glass.
With some of the sanded commercial mold mixes, I've had trouble getting particles of sand embedded in the surface of pate de verre unless I use this method. I suspect it's because I'm pushing powdered glass up against the surface of the mold, it's abrading away the topcoat and loosening the sandy grains, so that when the glass starts to compact it can pull in and capture the grains. For some reason the thin coating of kilnwash seems to be enough to prevent this--maybe it helps the glass slide past, I dunno.
Was that what you were looking for?
Last edited by Morganica on Thu Sep 10, 2015 3:05 pm, edited 1 time in total.
For all the precision I use in the rest of my mold mix calculations, I gotta admit I get all loosey-goosey on that one. It's literally as many handsful as I feel it needs to look right.
Typically, though, it's just enough to cover the top of the mound I'm sifting into the water, so probably about 5% or less of the total weight.
Alumina hydrate is pretty neat stuff as it has a good tendency to coat materials it comes in contact with. Since alumina is the best separator, this coating works. Alumina is not the same weight as silica, by volume, but essentially you are replacing some silica with alumina.
So, if you are devising a recipe, you can replace a percentage of silica with alumina.
Bert, can you give me the proportions of Plaster:Silica:Alumina I might start experimenting with? A starting point. Pete
If your mix is 40% plaster - 60% silica, try 40/50/10 plaster/silica/alumina or something... EPK is good, that is a high alumina clay. diatomaceous earth is good. You can keep substituting 10 parts of silica with other stuff.
I thought I would tack my question onto this thread as it directly relates. I'm getting back access to studio after years of being away and will be focusing on casting using bullseye frits. I'm having trouble sourcing small quantities of #1 Casting Plaster. For the formulas mentioned what might be a good alternative plaster component for my use and in ready supply around US (I'm in Richmond VA)?
#1 pottery plaster is almost always the easiest to find, and it's good, serviceable stuff--it's the basic plaster used in much of today's glass casting, especially beginning classes, and you can get it from just about any ceramic supply place.
You can also buy "stones," i.e., dental plasters that are a bit harder and usually have lower shrinkage rates so less tendency to crack. Hydrostone is probably the most common of these that you'll find in the US and it's probably available at the ceramic supply places in the mid-Atlantic. About the only plaster I really wouldn't recommend is something like plaster of paris; it's just too soft and shrinks too much, so molds made with it are too prone to cracking during a casting firing.
Otherwise, most of the common gypsum plasters and cements will work as refractory investments once you've done some experimenting with proportions. (there are only really three components to any mold mix: binder (the "plaster"), refractory filler (the silica/alumina/talc/diatomaceous earth/bentonite/sand/etc), and "additives" intended to change the mold characteristics, such as grog, fiberglass, organics, etc.)
Your goal (usually) is to get a mix that will dry without cracking, expand smoothly with the glass on the heat ramp up, stay strong as long as the glass is flowing/soft. Then it weakens as the glass starts to contract and finally--poof--degrades and begins to crumble as the glass cools below the strain point.
So...if you know how your binder responds to the heat, that tells you a lot about what filler to use, and in what proportions. It also helps you figure out what additives to use, and where they should sit relative to the mold surface. I tend to find that my mold mix proportions vary according to the form factor of glass I'm using and the casting method I employ (soda-lime pate de verre, especially if I'm primarily using powder, does best with a very hard mold mix with a carefully prepped hi-release surface, at least if I want to minimize coldwork, but I can be much less fussy about a transparent casting done with lead crystal billet).
There are a couple of really good books on refractory mold-making, if you're interested. Angela Thwaite's is really good (it's available on Amazon), as is Jim Kerwin's.
