casting and contraction of glass Q

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rodney
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Joined: Mon Mar 10, 2003 12:05 pm
Location: new mexico
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casting and contraction of glass Q

Post by rodney »

when casting a vase, what is the material used for the interior of the vase mold,,,,,wouldnt a plaster/silica material be so strong as to make the glass break when the glass contracts around it after the annealing stage,,,also, how is the best way to get rid of this mold material, so the glass wont be damaged during removal

thanks
rodney :D
charlie holden
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Joined: Thu Mar 13, 2003 8:26 pm
Location: Atlanta

Post by charlie holden »

It depends on the size and thickness of the vase and the type of glass. When I'm casting a hollow form I mix softer plaster for the plug than I do for the outside of the mold. I usually use a ratio of 1.75 or 1.8 to 1, by weight, of dry plaster /silica to water. For the plug I decrease the amount of plaster/silica to a ratio of 1.7 or 1.65 to 1. More water equals a weaker mold which means that the glass can crush the plug as it contracts.

You can also add the normal things to make the plug porous.

If the walls of the glass hollow form are thick enough, or if the size of the form is small enough that there isn't much contraction, the walls might be strong enough to crush the plug in any case. Likewise, soda-lime glass is stronger than lead. I know of someone that had great success casting Bullseye bowls then tried the same process on some lead crystal and all the rims cracked until she softened her plugs.

The important thing to remember about removing plugs after firing is to never let them get wet. They will expand and crack the glass. You just have to chisel them out the hard way.
watershed
Posts: 166
Joined: Wed Mar 12, 2003 1:44 am

Post by watershed »

Double on the "don't get the plugs wet", boy that annoyed me.

But on clean out, if you are carefull, a masonry bit can help a lot. Just swiss cheese the plug, then you can easily dig the rest out.

Greg
Ian
Posts: 30
Joined: Fri Sep 12, 2003 2:44 pm
Location: Amalinda, South Africa

Post by Ian »

I think that this may solve your problem with plugs. When adding to or repairing porcelain crown and bridge work I make masks (plugs) to protect the bridge and then I can weld new or extra pieces to it without damaging the bridge The secret is to use a 50/50 mix of Plaster of Paris and fine pumice Once this mixture is heated up it becomes friable. Once it has cooled and is easy to dig out
Hope this helps Pumice is available at any dental supply house. Just ask for polishing pumice.
Ian
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