WarmGlass.com

> Lauri where do I get the Hartgips? Hardware store?

I think so. Brand names like Supradur, Primodur... or
Knöllbach hartgips.

Most hardware stores stock "building plaster" that has some filler and is very weak. Germans use the term Hartgips,
but mold plaster or casting plaster are much the same.
-lauri

I have just been given a recipe for mold making that consists of 1 part by volume of each of the following: 1 part flint or quartz plus 1 part fine casting plaster plus 1 part water. Add dry ingredients to water and stir well by hand to eliminate any lumps then add 2 Tbs of glass fibers or vermiculite per litre. Have not used it yet so do not know results it may produce. Carolyn

Thanks for the info. Someone else (actually GLS a terrific German art glass store) told me that they used fiber paper and added that to their secret recipe for making molds. They crumple & tear up the used fiber paper that they otherwise would have discarded and add it.

Just a heads up. I draped a sheet of glass over the mold consisting of the potters plaster, the SiO2 (quartz), and the grog. The mold withstood the temp. up to 1400 degrees for well over 30 min. The final result was that I had to chisel the mold out of the glass. Undercuts? Don´t think so. Wrong plaster? Don´t know. Don´t know what else I could have done wrong. Either way I did mention earlier that I would post the results and here they are. The slumping didn´t go quite as I had anticipated either. Most of the detail was lost. Originally I fired at 1350 degrees Fahrenheit for 45 min. and then gradually worked my way up to 1400 degrees. It appeared as though the detail just did not want to come through. Should I have fired longer or is the 3mm glass not thin enough to get intricate details. The object was a mini face. About the size of a small doll face but quite a bit larger than that of a barbie doll. The eye sockets were a tad larger than the size of a pea. They barely even showed up as well as the mouth. I have to confess that this has been my first time slumping ever so it was quite exciting during the firing and disappointing after it all. The kiln has the elements on the sides as it is a front loader. I´ve heard that that slows down the slumping.

3 mm glass has very littly weight to push itself into small details. You will need to go higher, longer or add another layer of glass. You are getting quite close to casting when you are picking up detail that small.

You don't say what type of glass you are using, at least not in your most recent post. If you are using float glass you will have to go higher still -- it is stiffer than Bullseye which is stiffer than lead crystal.

For very small hollows, like holes for eyes, you may be having a problem with trapped air. The glass seals against the rim of the hole and the trapped air won't let the glass slump in. You can either drill very small holes in the bottom of these depressions or cast the plaster with small straws, like from a broom, running into all the low spots. The straws will burn out. This will provide an air vent. Some people mix chopped newspaper or cellulose, (from home beer making suppliers) into their dry mold mix. This will make the plaster a little porous and reduce venting problems.

As to why the plaster stuck, I don't know. Was it actually bound to the surface of the glass or just trapped in between the walls of the glass? If it's the first, make sure you mix the dry ingrediants very well before adding water. This gets the plaster to coat all the quartz and grog well and helps the release. If the plaster was just trapped try to hollow out the mold when you pour it by pushing a ball of newspaper into the back. The newspaper will burn out and leave a hollow mold. Also, you can make a softer mold by using more water.

ch

Thanks Charlie for all of the helpful info. If I use two sheets of glass (Bullseye) the weight will make more detail? I thought that it would be the other way around but I will give it a try. I will also look at the casting section of this website and see what I come up with thanks to your tip. The plaster did come off of the glass easily once I chiseled out the mold so I guess it was stuck between the walls of the glass. Will try all of your suggestions within the next few days.

Hallo Gator Girl

are you from Florida originally. We are from Darmstadt Germany, living now in Cape Coral Fl. Quarzsand is Silica and you might even get it im Baumarkt - Sand fuer Sandkaesten - is very good for making molds. A little bit mold hardener and you can create some unusual shapes.

Have you tried Tiffany Glas Kunst in Schloss Holte? their internet site is http://www.tgk.de

Will you move back to the us some day?
petra

Petra I hope that you received the PM. The computer was acting quite strange as I submitted it. If not just let me know and I will send another one your way.
Thanks,
Kim