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Is there anyone out there that should stop me before I try smasing up broken Cobalt Blue wine bottle for plaster silica open faced mold castings.
I just thought it would be a little cheaper than buying Cobalt from eaither spectrum or BullsEye for big bucks. I trid cutting them up into slices (flattened)and they fused together, well tacked together nicely !

Tyler

Tyler

Bottle glass is the worst possible glass to cast with. It is formulated to freeze up quickly when the bottle is formed with molten glass. In other words it's flow characteristics are minimal. If you have access to a glass furnace, you could melt the bottle glass and add fluxes to improve those characteristics, but now you've spent time and money to do so.

The store bought casting glasses will much more likely bring you success, unless the look you are going for is shards of glass stuck together (probably devitrified).

I say what the heck, give it a shot. Experience is the best teacher. Make sure you look at it with a polarimeter before you sell it so you know there's no incompatibility stress in it.

ch

Bert Weiss wrote:
unless the look you are going for is shards of glass stuck together (probably devitrified).

Those were my results every time I tried with every method I could come up with. Not trying to discourage you; I'd love it if you made it work and would tell me how simple it was so I could slap my head and say Duh! Yo!

I'll try it post my results when I do !

I'd say if I fire it really hot it might work (1675 or so), looking at my tacked Bottle pieces that I already tested to tack !

Tyler Frisby wrote:I'll try it post my results when I do !

I'd say if I fire it really hot it might work (1675 or so), looking at my tacked Bottle pieces that I already tested to tack !

Realize that plaster/silica is not happy at any temps above 1400. 1675 is way above 1400. There are reasons people spend big bucks to cast with Gaffer glass or Schott. The lead lowers the maturing temps making for more reliable castings.

Bert, what if I mixed in some garden style Vermiculite in thier I've seen that done here at a local Glass shop , they say it makes you P/S mold last for more firings.

your right though Gabber is probably a better way to go . thanks

Tyler Frisby wrote:Bert, what if I mixed in some garden style Vermiculite in thier I've seen that done here at a local Glass shop , they say it makes you P/S mold last for more firings.

your right though Gabber is probably a better way to go . thanks

Tyler

Mold mix recipes is a subject that I spent loads of time working on about 6 years ago. At the time I had to give up the project because I had killer bubble problems. I now feel the I might know how to solve those issues and I might just get to a new round of casting.

Additives are a good thing. I spent a lot of time consulting with various plaster experts. I learned a few basic things. First I weighed all the ingredients and made my recipes by percentages. The most impoprtant factor is viscosity. A thin mix is easy to pour and get a nice looking casting, but it is weak and problematic. A thick mix is difficult to pour with a good looking surface.

My hindsight says that I would work on a technique with a double pour. The face coat would be thin enough to get the nice surface and protect the glass from sticky particles. I would make a backup mixture that contains grog (groundup firebrick). The grog makes the plaster stronger and allows water to find it's way out of the matrix. Grog sticks to glass so it must be seperated from the glass.

There is a learning curve here. You have to get a feel for the right mix.

Bert Weiss wrote: Realize that plaster/silica is not happy at any temps above 1400. 1675 is way above 1400. There are reasons people spend big bucks to cast with Gaffer glass or Schott. The lead lowers the maturing temps making for more reliable castings.

hi Bert.

my question is a bit off topic.
i use a dry 50/50 mix of gypsum plaster and silica sand for float glass slumping. will this dry mix still be happy above 1400 F?

thank you

nelson

Nelson Tan wrote:

Bert Weiss wrote: Realize that plaster/silica is not happy at any temps above 1400. 1675 is way above 1400. There are reasons people spend big bucks to cast with Gaffer glass or Schott. The lead lowers the maturing temps making for more reliable castings.

hi Bert.

my question is a bit off topic.
i use a dry 50/50 mix of gypsum plaster and silica sand for float glass slumping. will this dry mix still be happy above 1400 F?

thank you

nelson

It will be happy enough but it won't soak up water and harden into a mold after you fire it. There is some water bound up in plaster even though it is a dry powder. This burns out around 350 F as I understand it. After that it is something else -- whiting I think. There's also some sulphur that burns out at some point. Ron Coleman is the guy that knows about all this chemical change stuff.

ch

charlie holden wrote:

Nelson Tan wrote:

Bert Weiss wrote: Realize that plaster/silica is not happy at any temps above 1400. 1675 is way above 1400. There are reasons people spend big bucks to cast with Gaffer glass or Schott. The lead lowers the maturing temps making for more reliable castings.

hi Bert.

my question is a bit off topic.
i use a dry 50/50 mix of gypsum plaster and silica sand for float glass slumping. will this dry mix still be happy above 1400 F?

thank you

nelson

It will be happy enough but it won't soak up water and harden into a mold after you fire it. There is some water bound up in plaster even though it is a dry powder. This burns out around 350 F as I understand it. After that it is something else -- whiting I think. There's also some sulphur that burns out at some point. Ron Coleman is the guy that knows about all this chemical change stuff.

ch

Nelson

With no added water you should be OK. I would worry a bit about the silica sticking. Test before you commit an important piece. I have used a powder mixture of alumina, diatomacous earth, and plaster over and over for a long time, with no problems. I avoided using silica because it is a dangerous particle and because silica can be prone to sticking. Covered with plaster the silica might behave just fine.

I believe that silica weighs more than plaster so a mixture that is 50/50 by volume is closer to 40% plaster / 60% silica. I'm not positive about that, but I think it works out that way.

Thank you Charlie and Bert.

What is diatomacious earth?

Nelson Tan wrote:Thank you Charlie and Bert.

What is diatomacious earth?

Diatomacious earth is fosilized diatoms that are ground up. It has various uses like an organic insecticide, and swimming pool filter. The swimmming pool filter aid is the least expensive source for it. It is a light fluffy particle, with sharp edges, that when covered with plaster, doesn't stick to glass, much.

thank you very much bert

Bert Weiss wrote:

charlie holden wrote:

Nelson Tan wrote:
hi Bert.

my question is a bit off topic.
i use a dry 50/50 mix of gypsum plaster and silica sand for float glass slumping. will this dry mix still be happy above 1400 F?

thank you

nelson

It will be happy enough but it won't soak up water and harden into a mold after you fire it. There is some water bound up in plaster even though it is a dry powder. This burns out around 350 F as I understand it. After that it is something else -- whiting I think. There's also some sulphur that burns out at some point. Ron Coleman is the guy that knows about all this chemical change stuff.

ch

Nelson

With no added water you should be OK. I would worry a bit about the silica sticking. Test before you commit an important piece. I have used a powder mixture of alumina, diatomacous earth, and plaster over and over for a long time, with no problems. I avoided using silica because it is a dangerous particle and because silica can be prone to sticking. Covered with plaster the silica might behave just fine.

I believe that silica weighs more than plaster so a mixture that is 50/50 by volume is closer to 40% plaster / 60% silica. I'm not positive about that, but I think it works out that way.

Hi.. from a newby here onn the board

I have used the plaster/silica compound up to 1500 with no problems, stcking, etc. You are correct on your proportions they work well. As I type I am firing another test uo to 1600 to see how it behaves. I am using float before I commit funds for good glass.

What are the proportions you use for the alumina/dia. earth/plaster compound. I would be interested in trying it.

I don't know how my mold compound would work for multiple castingds as my woodcarvings that I use for molds have undercuts so the molds are destroyed after one use.

TH