Plaster question

"Sam" wrote in news:75s_b.110483$ snipped-for-privacy@twister.tampabay.rr.com:

Newly cast plaster moulds take yonks to dry out.

I put my latest ones on top of the kiln to give them a hurry up.

If you have in fact used to much water all that does is to make a weaker plaster that will not last that long but will really suck the water out of the slip.. When making up plaster for moulds it is very important to weigh the plaster and make sure your water volums are correct. On some bags of plaster the reccomendation is also to weigh the water

John W

John, I am curious.. what ratios do you use. It has been recommended 1 H2O to 1.3 plaster, but it seems weak to me. Yes, I measure both plaster and water ( especially for multiple part moulds.) I also use paper plaster. Here, in sunny,summery Perth, Australia it takes about a week to dry a mould. Patience is something we learn when potting. (I guess) Andrea

"A&V" wrote in news:403aa87c snipped-for-privacy@news.iprimus.com.au:

I use the ratio that is recommended on the bag of plaster

I havn't experimented with other ratios.

I was up in Alice Springs in Decewmber, 42 degrees. I about died.

Regards

John Auckland New Zealand

in article 75s_b.110483$ snipped-for-privacy@twister.tampabay.rr.com, Sam at snipped-for-privacy@newnews.com wrote on 2/23/04 1:00 PM:

Patience is a virtue.

Don't know what to do with the molds but my comment on plaster in gereral would be, that they vary so do, as someone else said, read the instructions. At university, they were cheap, so for sculpture and mold making, we used building plaster. It has no instructions and we were taught to judge amounts by the way it mounded in the water. It got hard fast which is much preferable when sculpting with it. I was amazed the first time I used pottery plaster when it took what seemed like an hour to harden. Supposedly pottery plaster is harder and therefore advisable. I've also heard there are ways to speed up the plaster set...hot water?

I believe there are FAQs associated with...alt.sculpture..rec.sculpture ..rec.crafts.sculpture??? that might help.

Elaine

Not being picky about words, but "set" refers to the hardening, but is not at all the same as drying. Plaster sets up in a few minutes or 10s of minutes, but can take many days to dry out afterward. I'm assuming your plaster set up OK, and you are just trying to get the pieces dry enough to use. If so, the oven idea is fine. In fact, you can leave the oven on with the door open a crack. Just be sure to keep the temperature below boiling.

(You probably couldn't actually get steam that damaged the plaster, but if you get the pieces *too* hot you will calcine the plaster back to its original pre-mixed state. 212F is safe, but (say) 312F is a problem if you let it go too long. I have calcined at 325-350F to rejuvenate "dead" plaster powder that was exposed to too much humidity for too long before I got around to using it. Saved a long trip to my pottery supplier, but definitely *not* recommended!)

The proper proportions for pottery or moulding plaster are 70 pounds of water to 100 pounds of plaster. ("Hydrocal" is only 45:100) I have found that when I use the "by eye" method of mixing until mountains form above the water, that I don't always get that much plaster in the mix. (This may be due to lack of experience with the technique.) I think that produces weaker final results, but it worked well enough for plaster drying bats.

Nowadays I keep two clear measuring cups. I weighed the amount of plaster to fill the first one to the brim. On the second one, I made a mark at the point for the amount of water weighing 70% of the first. (It would be even better to make one that holds exactly that amount.) Now I can just put so many measures of water in the mixing container, and add the same number of cups of plaster to a separate plaster dumping bucket. Then I can sprinkle from there into the water with good control over the process.

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

Right, that was actually what I meant. I was not sure if the dampness was a result of not being done with the chemical process of setting, or if it was just wet with water and I could dry it in an oven. Your complete response answered that question for me. Thanks.

I think my problems stem from following directions. They say 1:1.75 water/plaster, but that leaves it soupy. My recent trials of 1:2 makes a more creamy mix and seems to set in a few minutes, warms up and then drys fairly well overnight.

Or maybe it was the change in the weather, water temp in my pipes... or maybe cosmic rays...

The ratios you mention don't look right to me. Are you talking about volume instead of weight? By weight it should be 1:1.43 water/plaster to get the specified 70% water. I just measured the volumes that I use to get the 70% weight, and the ratio comes out to 1:2.46, so you are still a little shy on plaster.... assuming your plaster and mine are the same density.

Another thing that can ruin your plaster is stirring it once it has started to set. You need to use cold water so it doesn't set too soon.

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

Yes, since I am using tablespoon and 1/4 cup amounts, I don't have a scale that is accurate enough to do weight.

I'll back up and regroup (I think I can get a gram scale), thanks for your help.