-1-  Can this be caused by silica that is too old?

I know that casting plaster has a shelf life (I’ve been told about a year).  I’ve never heard of silica going bad.

-2-  Is there something I can coat the plaster mold with to prevent this?

Not coat – but adding 3% alumina hydrate to the plaster silica mix will give you a cleaner release.

The other problem I am having is with the clay that I am using to make my mold.  After my plaster/ silica has set, the clay sticks to the mold.  I guess you can expect a little sticking, but this is so bad that I have to scrape almost the entire inside of the mold to get the clay out.

The longer you wait after the plaster sets the more difficult it is to remove the clay. The clay should be removed within a few hours after the plaster sets up.

-3-  Is there a special clay that you can use to prevent this from happening?    or

-4-  Is there a release agent to put on the clay to get the sculpted clay to come out of the mold without leaving behind clay?

A waterpik (the kind people use on their teeth) works well.  You will get soaked though :)

-5-  What can I do to improve the surface of the glass?

Try the alumina hydrate – but be aware that most people who cast also coldwork — and there is a reason they call it coldwork.  Depending on the form you can sometimes fire polish.  The trick is to heat the piece slowly past where it can thermal shock and then go as fast as possible to fire polish temp (no hold) and then as fast possible to anneal soak.  It is like trying to wave a hair dryer past a scratched piece of wax to polish the surface but not give the heat a chance to penetrate deeply enough that the piece distorts.

My last problem is that I used too much glass and it ran over the top of the mold.  I used water displacement in order to determine how much glass to use, and I thought I was being very precise, but I ended up with too much glass.

-6-  What is the best way to determine how much glass to put into the mold?

Fill you mold with rice and then pour the rice into a container where you can measure the volume. Then use the fused glass casting calculator on this site to determine the weight of the glass needed to fill the mold.

Hope this helps,

Helios Kiln Glass Studio


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