mold additives

[Dick Ditore]

Hi all. I have been using a simple plaster, silica mold mix with good results. I have read about additives such as fiberglass mesh for bigger molds, and for more durability. What exactly do you add, and in what quantities. Finally where do you get the stuff? By the way, molds would be used for slumping.

Thanks,

Dick

[charlie]

Hi all. I have been using a simple plaster, silica mold mix with good results. I have read about additives such as fiberglass mesh for bigger molds, and for more durability. What exactly do you add, and in what quantities. Finally where do you get the stuff? By the way, molds would be used for slumping.

Thanks,

Dick

you can get chopped up fiberglass strands in lots of places, as it's used a lot.

cement additive. call up a redimix place.

building plastic boats. probably lots of boat places in san diego.

chopped mat is also used to rebuild fiberglass auto bodies or swamp coolers, for instance. you can get that at home depot in the paint section. or you can try an auto paint shop.

[Bert Weiss]

Hi all. I have been using a simple plaster, silica mold mix with good results. I have read about additives such as fiberglass mesh for bigger molds, and for more durability. What exactly do you add, and in what quantities. Finally where do you get the stuff? By the way, molds would be used for slumping.

Thanks,

Dick

Dick

I add a material that I get from a perlite factory. It is unpopped perlite and is a waste product. If you have a perlite factory near by check it out. Another material that does the same thing is grog. Grog is ground up fire brick. It definmitely sticks to glass so you need a good slip coat backed up with a grog mix. The grog allows moisture to find a pathway out without cracking.

I learned to make plaster mix by weight not by volume. I would try 40% plaster 40% siica and 20% grog. Ths slip coat can be 60% silica 40% plaster. I think that is comparable to 50/50 by volume.

Bert

[Dick Ditore]

Thanks for the info. I will try some experiments.


Dick

[Gale aka artistefem]

Hi Dick........will your plaster/silica slumping mold be fired once? Or would you like to reuse the mold several times beyond just your initial project?

Even with the mold mix additions, plaster/silica mixes have a short use life. They're just too soft for the long term. Which of course is the beauty of them when you need to break the mold away from full surround or undercut castings.

With your open face slump mold, you might consider making the original mold in a low grog light colored (no staining oxides) clay body. I've only recently started making molds like this for open face slumping and casting and they're holding up - still fire-worthy after 5+ firings.

The glass artist who turned me onto this alternative mold material has a collection of hand built clay molds he's been firing in for years.

Plus even the commercially available slip cast molds that I've purchased over the years have held up pretty well. You know - the ones you can buy from the greenware ceramics/paint your own pottery businesses.

[Paul Tarlow]

Hi all. I have been using a simple plaster, silica mold mix with good results. I have read about additives such as fiberglass mesh for bigger molds, and for more durability. What exactly do you add, and in what quantities. Finally where do you get the stuff? By the way, molds would be used for slumping.

Thanks,

Dick

Another addative (learned from Ruth Brockman) -- not for strength, but for less 'plaster scum' is 3% alumina hydrate. I don't know if you need it at slumping temps but it works well at casting temps.

- Paul

[Jerry]

And, in addition to all the advice you've already gotten, I learned a formula from Delores Taylor who is probably lurking nearby. Rather than chopped fiber or any of the other things, which work well, add 1/3 as much talc as your plaster/silica. The talc makes a smoother surface around your model and is a much more durable mold. It still won't work more than once but the talc stops cracks; even the hair lines, and makes demolding much more fun! I needed a heavy hammer and chisel on my last one! Of course, the secret to success with plaster/silica is to dry the mold well.

Good luck,
Jerry