Cold Casting Stone

[Jean]

I HAVE GOOD RUBBER MOLDS AND WOULD LIKE TO TRY MY HAND AT CASTING DURASTONE OR SOME OTHER PRODUCT TO COME OUT WITH A MARBLE FINISH OR TERRA COTTA OR WHATEVER.
IS THERE SOMEONE OUT THERE IN CYBERSPACE WHO IS FAMILIAR WITH THIS PROCESS? DO YOU HAVE ADVICE FOR ME? THANKS!

[fritchie]

Jean - Cast stone of the type you discuss is fairly well-known, though not entirely common. The binder typically is a resin such as epoxy or possibly others.

I have seen, though not made myself, epoxy casts “filled” with ground marble or dry powdered clay - the latter gives a sort of ceramic look. The small number of epoxy casts I did were “filled" with chopped glass fiber, an agent which serves both as “fill” and as a strengthening agent. I put quotes around the word “filled”, because these resins always are “filled” or bulked with some inert agent as a means both of stretching the expensive resin and also of strengthening the final cast.

[davphil]

Jean, don't know how you are coming along on this, but I too am trying to cast with resins. Problem is that resins can melt your latex molds very quickly. If you intend on making more than 3 or 4 casts, wekk it may be an uphill battle. I usually cast in plaster and soon I will be using hot wax in the molds to make for casting. Just make sure your molds have a lot of release agent in them. Let me Know. Dave

[Sean McLean]

Here in Australia there is a gypsum-based product, manufactured in the USA I think, called "Hydrostone". Not sure who the original suppliers might be. Comes in 20 Kg bags. Gypsum is the same technology as cement, but this is a highly refined and very fine grained powdered product that's almost "pure white" - really slightly off-white I guess. Stays pourable for about 8 to 20 minutes, depending on temperature and degree of agitation in mixing. Rapid mechanical mixing (as with a paint mixing attachment mounted on an electric drill) will cause quicker setting. There are no "fillers" to my knowledge; purely gypsum technology - so no noxious and potentially carcinogenic polymer fumes or particles.

Sets in about 20 minutes (depending on temp. etc) to about 60% of maximum compression strength at 6,000 psi (!!!) - full strength is 10,000 psi. Sets very hard and has a solid "feel" rather like "solid rock" as if it was an African carving from soapstone. Most models may be demoulded at around 20 minutes, others may require full curing. Reaches full strength after about three hours. While still "green" (before final cure) it may be readily sanded (to remove seams in multi-part moulds or to fix blemishes. Bubble holes or mould imperfections may be fixed by back-filling with a little extra hydrostone. Once fully cured, the material may still be finished, but it is somewhat harder to achieve. A simple, small kitchen paring knife is the most useful tool I have found for many finishing operations. Grind the blade down until it is very short for easy control. Finishing plaster may also be used over small areas to disguise imperfections. But a pristine mould should produce a near perfact mould with models suiting glove moulds, and a darned good result with two-part or multi-part moulds.

The mix is calculated simply by calculating the water component as 32% by weight (just call grams millilitres if working in the metric system but the USA is stuck with the absurd and archaic "Imperial" system - so bad luck and get your legilators to modernise!). For example, if the hydrostone required weighed 1000g, you would need 320g of water (32%), so just use an accurate kitchen measuring jug to weigh out 320ml. This of course, is because for all intents and purposes, 1ml of water weighs 1 gram. Or you can weigh the water of course, but a measuring jug is a quick and easy method - no need to allow for the weight of the container.

I guess you could track down Hydrostone just by a simple search on the Internet. Oh, and Hydrostone takes paint. Can be spray painted or hand painted. We use Estapol as a finish to provide a polished look and protection of the paint. Hard scratching will dislodge such a layered finish, but most models or figurines are simply placed on display and so the finish can last indefinitely.

Moulds may be of any kind, including latex (for glove moulds) and silicon (for either glove or "solid" moulds). RTV 585 is useful. Latex is great though - perfectly natural, no toxic catalyst and much, much cheaper. Have made hundreds of pieces from a single solid silicon mould, and dozens from glove moulds either of latex or silicon. Both types may be patched as they wear, but this usually creates distortion on the surface of the piece being cast - hence increasing the finishing effort (and time!) required. There comes a point when it's better to throw out a glove mould and use a pre-made new one. Cheap anyway, whether silicon or latex, but especially cheap if latex. But solid moulds cost heaps since silicon is so expensive. Ouch! String them out as long as you can! When "dead", chop 'em up and use the fragments as "fill" in the mix for your next mould. Silicon binds very well to silicon (and almost nothing else!). Saves money to recycle!

Have fun!