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I just finished a clay positive and I'm now preparing to pour my plaster mould for open face casting.
I am going to use a splash coat of 1 part pottery plaster and 1 part 200 mesh silica.
The info I have says to use "Gerry's Mold Mix" but doesn't give any recipe for this. I know that I have to use something stronger than the splash coat to survive the casting.
I have #120 silica sand on hand and some finer silicone carbide sand for sandblasting on hand. Will either one of these work to give me a stronger mix that will hold up?

I would not do a splash coat. Esp if you are going with a different mix for the body of the mold. You might get flaking, and that would be bad. The 50/50 will do fine with chicken wire reinforcing. You WILL want to lightly sponge the entire inside to remove the clay film, or it WILL show up on the piece.

If you are out of 200 mesh, and must do a splash coat, make sure that the splash coat is soaking wet, when you pour the rest.

Greg

Greg,
I have plenty of plaster and #200 mesh silica, but the info I read said to make a thicker bodied plaster with coarser, stronger material after applying a splash coat of about 1/8" thick.
So, should I nix the splash coat, reinforce my pour with chicken wire?

That's what I do. Esp with an open face, you should have minimal cracking, with proper drying.

I just saw some nice molds come out of the oven today. Steel casks, 60 silica/40 plaster, but contained by the steel outer shell.

Also if you are paranoid, stack a couple of hard brick around the edge of the mold, in case it wants to go flooeee.

Greg

Greg,
I like the idea of putting a steel collar around the perimeter. Obviously, if you are doing that, there must not be a problem with the plaster and steel expanding and contracting at different rates, right?

For open face casting you don't need anything more than a monolithic pour of 50/50 plaster/silica, as long as you aren't casting glass that is deeper than a couple of inches. Mix the dry ingrediants very well, (I mix mine for upwards of 45 minutes) and use a ratio of about 1.7 to 1, dry mix to water by weight.

Certain plaster cements and dental plasters are stronger than pottery plaster. A mix of particle sizes provided by things like grog or ludo help in packing the mix tightly. Some fiber like ceramic fiber or borosilicate fiberglass helps bond the mix. Something organic that will burn out, like ground paper or cellulose, helps make the mold porous so that water can burn out, and along with a face coat, they can provide a break in the mass of the mold where cracks might stop running.

All of these things will make a mold stronger. But it's only when you start pouring deep pieces that you need to really worry about them, or about reinforcing molds with wire or ceramic shells. Everybody has different mixes and methods. It's amazing, really, what you can get away with.

Dean Hubbard wrote:The info I have says to use "Gerry's Mold Mix" but doesn't give any recipe for this.

"Gerry's Mold Mix" is a proprietary mix, instead of a recipe. A search for the name in the Old Archives will turn up information on it.

Thanks Charlie,
I found a good thread in the archives, where you mentioned the chopped up paper technique. I take it that you have an automitic mixer if you mix the dry parts up that long. Thanks also for advising me to continue with my testing before I get my bell kiln built. I can see now, that I will need to come up with another type of mould for doing the large open faced cast panels, like a carved board or carfefully placed pieces of fiber paper. I'm going to need to read up on open faced sand casting as well.

Jerry, thanks for clueing me about Gerry's Mix.

No probs with the steel outer shell. The guys in the shop swear by them, and I'm watching the results carefully. The other "new thing" is using expanded metal instead of chicken wire, for a reusable, reinforcement. These molds have not been removed yet, so I don't know how the metal will stand up. But it worked well as a one timer, though a little more expensive than chicken wire.

Greg

You're not going to find much written about sand casting in a kiln, other than here. It is just now sort of moving into public consciousness. It has been done for a long time by a few slumpers that work on large, architectural scale, mainly in Australia. But it has been held close as a somewhat proprietary technique. Bert's post in the photos forum is one of the first public demos and discussions.

charlie holden wrote:You're not going to find much written about sand casting in a kiln, other than here. It is just now sort of moving into public consciousness. It has been done for a long time by a few slumpers that work on large, architectural scale, mainly in Australia. But it has been held close as a somewhat proprietary technique. Bert's post in the photos forum is one of the first public demos and discussions.

Yeah, Graham, (and many of his students) had done it for decades when I met him in 1999. It's quite usual in Oz. Brock

Charlie,
I looked at Bert's thread on sand casting the coffee table, awesome! I think sand and bentonite might be a direction for me to explore.
On a larger project for architectural purposes do you think there would be a problem with the weight of the glass causing problems with the pattern in the sand?
I assume this is all done dry with glass, which leads me to believe the sand could be used over again by just forming a new relief pattern.
I've looked at some sand casting sites using green sand to cast metal and they get the sand wet first, just like casting hot glass in sand. Could this type of approach (not water) be used in a kiln with some sort of binder to keep the sand firmly in place? I would think that oil would burn off too quickly.
I would like to see an involved discussion on sand casting.

Just to remind folks. There's quite a discussion in Glass notes on Sand casting, in in out of the kiln. Jose Chardiet uses the method extensively in his work, and has since '83.

Greg

Dean Hubbard wrote:Charlie,
I looked at Bert's thread on sand casting the coffee table, awesome! I think sand and bentonite might be a direction for me to explore.
On a larger project for architectural purposes do you think there would be a problem with the weight of the glass causing problems with the pattern in the sand?
I assume this is all done dry with glass, which leads me to believe the sand could be used over again by just forming a new relief pattern.
I've looked at some sand casting sites using green sand to cast metal and they get the sand wet first, just like casting hot glass in sand. Could this type of approach (not water) be used in a kiln with some sort of binder to keep the sand firmly in place? I would think that oil would burn off too quickly.
I would like to see an involved discussion on sand casting.

Yeah, it's done dry. The bentonite is used to provide smaller particles so that the sand packs well. I've used plaster (still dry) instead of bentonite at a ratio of about 20% plaster. Plaster is sifted on the surface, (I've heard you can use bentonite for this too), as a release. You don't have to use olivine sand either. Cleaned silica sand is fine, though it doesn't quite pack as well. And it is used over again until the amount of plaster or bentonite gets too high. Then you have to mix in more sand.

You can't get the same quality of impression that you can casting hot glass into damp sand, but it is pretty surprising how much detail you can get. It's funny how the sand doesn't move much. I think it is just because the glass moves so slowly. The weight of the glass on the sand is a function of the thickness of the glass, not it's area. It's a question of pounds per square inch. So it can be translated up to as large a piece of glass as you can fit in your kiln.

For some pictures of sand cast work by Warren Langley see the link below. He's from Australia and has taken it farther than pretty much anybody else. He combines some other techniques with sand impressions. He uses a lot of glue and paint on his pieces as well.

http://warrenlangley.net/WorkinGlass.htm

Charlie,
I want to shelve these plaster moulds and go get some sand!
I went to Warren's site. did you see that piece in his available works section called the Vessel? How in the world did he achieve such a sharp corner at the surface transision point?
Thank You so much for bringing sand casting into view for me! I gotta tell you, I am totally jazzed about this tehnique!!!!!
I posted to Bert's First Pic thread on sand casting, to keep that thread going instead of starting a new one.
I don't mean to intermix my topics here, but as I recall on my kiln thread, you mentioned that you were considering going to an open metal grill under the brick, to dissapate heat better, for sand casting. Right?

Dean Hubbard wrote: How in the world did he achieve such a sharp corner at the surface transision point?

This looks like a simple drop mold made with 1" HD board with a hole for the drop.

Bert Weiss wrote:This looks like a simple drop mold made with 1" HD board with a hole for the drop.

Bert,
That is what I thought too, when I first looked at the thumbnail picture. Then I clicked on the thumbnail, to get a closer look. It appears to have a very sharp corner at the transision point where the drop starts. I have only attempted the drop ring one time on a small test sink and had success, but I sure didn't get a transisision at the drop point like that.

Dean Hubbard wrote:Bert,
That is what I thought too, when I first looked at the thumbnail picture. Then I clicked on the thumbnail, to get a closer look. It appears to have a very sharp corner at the transision point where the drop starts. I have only attempted the drop ring one time on a small test sink and had success, but I sure didn't get a transisision at the drop point like that.

I may be missing something, but the Vessel image I saw looks like a fused or cast panel wall mounted above a fused and slumped or cast boat shape sitting on a shelf.

Bert Weiss wrote:

Dean Hubbard wrote: How in the world did he achieve such a sharp corner at the surface transision point?

This looks like a simple drop mold made with 1" HD board with a hole for the drop.

If I know Warren, the black boat was slumped over a pile of sand, cold worked to a flat rim and glued to the big white panel.

The open grill under the kiln brick is to both dissipate heat and to avoid warpage. Steel plate wants to bend when one side gets hotter than the other. This is the stuff I've seen in other kilns:

http://www.mcnichols.com/products/bargrating/