hi guys, been some time but enjoying reading the forums again and need to ask aobut a problem i am having with plaster molds.
I made some latex rubber molds of portals and retaining walls as well as some rocks from the yard. I spent the extra (alot extra) to purchase a few boxes of lightweight sculptamold (sp?) which is new to me, in the past, like 20yrs ago, i used reg plaster for this work.
Well, on my first few attempts with my new home grown molds i found after i released the casting that the sculptamold near the mold, ie - the part of the casting with all the lovely detail, was “soft”. if i rubbed the detail lightly with a finger it came off. the majority of the casting is hard but again, the area that counts is worthless. I dont beleive my mix was bad and it certainly was given time to set. I may have had some talcom powder residue in the latex mold as i followed someones advice on this score to preserve the molds etc. could the powder residue act as a dessicant on steroids?
i also used the sculptamold on a mold made from plastic (WS tunnel liners) and i didnt have the issue?
finally, please steer me straight, does warm or cold water (or vinegar?) add working time to plaster. i’m quickly remembering the days of “workin real fast” with this stuff.
Usually the warmer the water, the faster the plaster sets up. If you’re plaster rocks are weak and you say the mixture was right, you may want to try a few drops of liquid detergent to the mold as a releasing agent instead of the talc…not sure if the talc was the culprit in your problem. Just finished a batch of rocks this past weekend with no problems and the soap did a good job of releasing the plaster from the molds. To me the plaster stuff from WS is in some ways weaker than the ol’e plaster paris since it’s lighter…to some extent my rocks are somewhat delicate but a coat of paint or hardner usually does the trick. Good Luck.
I think you’ll have better results with Hydrocal rather than Sculptamold. Hydrocal is really made for casting, while Sculptamold is made for “sculpting,” or applying to a surface by hand and working in the surface details like gullies with hand tools.
Hydrocal works great on my molds, both the store-bought black ones and my own creations from Latex.
I have always used regular plaster of paris when making thick castings like tunnel portals and retaining walls, etc. For thin sections like buildings I use the expensive stuff,(Hydrocal).
Hot water is a no no as it accellerates the hardening and setting process. Setting time for both plasters can be extended by adding some vinegar. It gives a little more working time, but I only use it for scenery appications since there isn’t a need to slow down the hardening of castings.
Both plasters can be poured pretty soupy and still set up well. Experiment.
I always use Hydrocal or, even better, 20 Minute Casting Plaster. You can’t find casting plaster in small quantities like Hydrocal, though. The smallest amount we could find was a 110 pound bag.