casting and contraction of glass Q

rodney · Post by **rodney** » Fri Nov 07, 2003 3:57 am

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

charlie holden · Post by **charlie holden** » Fri Nov 07, 2003 12:25 pm

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 · Post by **watershed** » Sat Nov 08, 2003 12:22 am

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 · Post by **Ian** » Sat Nov 08, 2003 4:54 am

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