WarmGlass.com

What is the best way to create a good re-usable mold for sand casting (crack free and smooth after drying) Or the most suitable way to repair any cracks / blemishes. I know there are several suggestions about additives to the sand but which one works the best?

Thankyou for your time.... XL

Your question is confusing as sand cast molds are not hard -- they're just sand and about 20% dry plaster sitting in the bottom of the kiln. They can't crack since they are granular. They aren't wetted. The sand is smoothed over after every use and new impressions are made in the surface.

ch

When I say re-usable I mean use re-usable several times without disturbing/removing from the kiln. Yes I agree this is not a "hard" mold but more of a crisp surface mold, however when using just sand (damp at first to form desired shape) from a thickness of 1"min to a max of 4" cracking has occured.
Hope this explains what I mean, any ideas?
Thanx XL.

hi XL

not sure if I understand your question. Anyway we do dry "sandcasting" too.

during our first try there are cracks around the perimeter of the impression. we have minimize cracks around the perimeter by making a flat "base" under the impression.

For example when making a circle impression we will make an inverted hat. the circle is pressed on the sand and the flat base or rim of the "hat" will flatten the cracks.

hope this helps
nelson

If you need to make a bunch (20+) of the same sand casting it would be more economical to make one sand casting- pull an alginate from it and have a metal mold created.

You end up with the look of sand with a perminent mold.

There are a lot of factors - undercut - 2 part - thickness - finish ......

You need to provide more details to get a better answer.

Either that or create a stamp that you can punch the desired shape into the sand multiple times.

The first impression is always the best.

excell wrote:When I say re-usable I mean use re-usable several times without disturbing/removing from the kiln. Yes I agree this is not a "hard" mold but more of a crisp surface mold, however when using just sand (damp at first to form desired shape) from a thickness of 1"min to a max of 4" cracking has occured.
Hope this explains what I mean, any ideas?
Thanx XL.

My question is are you trying to just re-use the sand or are you trying to re-use the impression? I don't know of anyone that tries to re-use an impression in a sand mold. Lifting the glass out always disturbs the sand, and it is so easy to smooth the sand out and make a new impression, that it isn't worth the effort to try to save it. Nor do I know anyone that dampens the sand before kilncasting, (it has to be able to hold the impression when it is dry).

If you want a re-usable mold you should go to a harder material. Bisqueware is one option. There is a commercial mix called Castalot that can be used up to 9 or 10 times. Some people make simple shapes with good old plaster/silica (you could replace the silica with sand), and never move the mold from the kiln shelf. Even though these molds crack they can be slumped into more than once if they aren't disturbed.

ch

The mold has been constructed using 1" fibre-board with curved top walls pinned together and placed onto a fibre-paper covered kiln shelf. Damp sand was pressed into the empty mold. The shape I wanted was a two way curve (like a segment of a sphere) this was achieved by dragging a glass template of the correct curve over the curved fibre board walls. This created a two way curve. Then the mold was hardened up by firing and removing all the moisture. This is when cracking in the sand occured Do you think if I dried the sand out slowly this would help reduce any cracking? if so how slow? The curve cannot be formed using dry sand.

Hope this explains it a little better.... Thanx XL

I just don't know how sand behaves as it dries out. I've never made a sand mold where I wet the sand first. In general slower drying is better.

If I were building this mold I would use plaster/silica instead of sand. You could make a master model exactly the shape you want the glass to be, then flip it upside down, build a box around it and pour the plaster/silica in to get a perfect negative mold. This takes practice and experience to do well.

If you don't have Keith Cummings' book "Techniques of Kilnfired Glass" you should get it. It covers the general principles very thoroughly.

ch