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Kabe

damp plaster

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Need some help here. Sometimes you do things and you seem to have the midas touch and all go's well and sometimes you don't and it all turns to, well, I call it the exlax touch. this was one of those learning experiences. I have some plaster of paris (that was a mistake in the first place, it is not as good as potters plaster, but I was in a pinch) I used it to poured some press molds for a chess set I am working on and the crap didn't set up right. I was a bit, let's say disappointed. I think I can get one good press out of most of the casts but they are real soft. If I can get one good cast I can use it to repour with better stuff. That way I will only have to remake the one piece that was lost. I think the plaster absorbed moisture from the air. I always double bag it in trash bags and keep it in a plastic container but no not this time.I got busy and left it open to the damp air of my dungeon basement studio. To the point finally. Can I put it in a pillow case and set it by a heater vent to dry it back out or do I just pitch it (with a smile) and go get some of the good stuff. I know plaster is not real expensive but I won't be using these molds again after I make the first set. What do you think. ain't clay fun Kabe

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Did you plaster mix have a high ratio of water in it, when I have a soft mix is I get a weak mold. I used to have my studio in a damp basement and kept my plaster in 5 gal lidded buckets and if they got exposed to dampness I would get little hard lumps and crusty surface from the plaster setting up. Did you let them dry and cure your casts completely before trying to use them? Denice

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Did you plaster mix have a high ratio of water in it, when I have a soft mix is I get a weak mold. I used to have my studio in a damp basement and kept my plaster in 5 gal lidded buckets and if they got exposed to dampness I would get little hard lumps and crusty surface from the plaster setting up. Did you let them dry and cure your casts completely before trying to use them? Denice

 

 

Thank for the reply I haven't tryed to use them yet, Like I said I hope I can get at least one cast out of them so I don't have to remake the pieces. I use a container that holds one lb of plaster and have plastic bottles that are mark of at 1 lb 2 lb ect. so I don't have to try and depend on the appearence or feel of the wet plaster to see if it is right . I mixed all three to the same ratio for what the size of the mold called for . so it should hve been ok Maybe the plaster of Paris is a slightly different ratio than the potters plaster. I am going to reweigh my containers and try a small pour to see if it is a ratio that was light on the plaster. Could be. will take any other advise that you may have Ain't clay fun Kabe

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Did you plaster mix have a high ratio of water in it, when I have a soft mix is I get a weak mold. I used to have my studio in a damp basement and kept my plaster in 5 gal lidded buckets and if they got exposed to dampness I would get little hard lumps and crusty surface from the plaster setting up. Did you let them dry and cure your casts completely before trying to use them? Denice

 

 

Thank for the reply I haven't tryed to use them yet, Like I said I hope I can get at least one cast out of them so I don't have to remake the pieces. I use a container that holds one lb of plaster and have plastic bottles that are mark of at 1 lb 2 lb ect. so I don't have to try and depend on the appearence or feel of the wet plaster to see if it is right . I mixed all three to the same ratio for what the size of the mold called for . so it should hve been ok Maybe the plaster of Paris is a slightly different ratio than the potters plaster. I am going to reweigh my containers and try a small pour to see if it is a ratio that was light on the plaster. Could be. will take any other advise that you may have Ain't clay fun Kabe

 

 

I have never figured that ratio of plaster to water, I learned to mix plaster in a commercial dental lab, they had you sift the dental plaster into the water until you had a mountain of dry plaster on top. This method has never failed me except if I run out of plaster and end up with a soft mix. Denice

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For a specific reply to plaster ratios & water needed: for No.1 Pottery Plaster, the ratio is about 70% water (by weight) to plaster. I have attached a chart below that I use not only for mixing, but for figuring out the right amount to use for the volume I need. The ROUGH RATIO IS 3 POUNDS PLASTER TO 1 QT OF WATER, as stated below in the chart. After I get everything set up and ready to mix, I hand strain the plaster into my mix bucket using a wire mesh food strainer. The process is quick, but gives the plaster a good chance to be absorbed by the water without creating lumps or clumps. After that, I will always mix slowly with a latex gloved hand, just to make sure all lumps have been eliminated.....Good luck on your next project. Les

 

PLASTER MIXING CHART FOR #1 POTTERY PLASTER VOLUME WATER WATER PLASTER CREATED IN IN IN POUNDS (CU.

INCHES) QUARTS POUNDS (70% CONC)

 

 

 

ONE GALLON= 4 8.33

ONE QUART= 1 2.08 3.0

 

 

 

80 1.0 2.08 3.0 88 1.1 2.29 3.3 97 1.2 2.52 3.6 106 1.3 2.77 4.0 117 1.5 3.05 4.4 129 1.6 3.35 4.8 142 1.8 3.68 5.3 156 1.9 4.05 5.8 171 2.1 4.46 6.4 189 2.4 4.90 7.0 207 2.6 5.39 7.7 228 2.9 5.93 8.5 251 3.1 6.53 9.3 276 3.5 7.18 10.3 304 3.8 7.90 11.3 334 4.2 8.69 12.4 368 4.6 9.56 13.7 404 5.1 10.51 15.0 445 5.6 11.56 16.5 489 6.1 12.72 18.2 538 6.7 13.99 20.0 592 7.4 15.39 22.0 651 8.1 16.93 24.2 716 9.0 18.62 26.6 788 9.8 20.49 29.3 867 10.8 22.54 32.2 953 11.9 24.79 35.4 1049 13.1 27.27 39.0 1154 14.4 30.00 42.9 1269 15.9 33.00 47.1 1396 17.4 36.29 51.8

 

 

 

ROUGH RATIO IS 3 POUNDS PLASTER TO 1 QT OF WATER

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Sorry about my post above. It is a nice Excel spreadsheet for mixing plaster that this system is not set up to handle. I'll try to post in a different format.

Regards, Les

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Thanks Les for the chart. Have always used a ratio of 3lb of water to 4 lb plaster (3 to 4) the chart is 2.77 to 4 so my ratio is a bit heavy on the water. It is really the first time I have had this problem that's why I thought maybe the plaster was damp so it had more water in it to start with. Still haven't tryed to dry it out. Most the time I use Potters Plaster not Plaster of Paris. I will up my ratio a bit and see if that helps. sorry it took so long to say thanks.

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Are you saying that your molds are still damp after 9 days? If you have a radiator place the molds on top of it when it is radiating heat; turn the molds from time to time.

 

Even after you have dried these molds it doesn't mean that the cast will be free of powdery, residual plaster. But you should get at least one good cast from them. If you have any ruined clay use it for making new models.

 

The problem with using plasters other than pottery plasters for mold making is, according to United States Gypsum, "much more water is required in order to get these plasters to a pouring consistency thus making soft and weak molds".

 

Pottery plaster is always best to use for mold making. It is best to save the Plaster of Paris for wall repairs.

 

 

 

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Life is short-to short for messing with bad plaster

When you redo this press molds use #1 pottery plaster-you can write a letter to United States Gypsum

and they will send you nice mixing slide ruler for mixing ratios

I have tried to screen those chunks out and reground bad plaster-take it from one who has tried-start fresh with the right stuff-its cheap and one needs to store it in airtight containers

Mark

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I use quite a few plaster molds. Lately I have been putting the almost dry molds on top of my electric kiln when I fire.

 

This seems to achieve maximum dryness. In the summer I put them on the cement driveway and solar :huh: dry them... heehee and that works really well.

 

I recently bought a couple slump/hump molds and they say in the directions which I actually read, to put them in the oven to cure them. I had no idea that there are different ratios of plaster to water for different size molds.

 

This reading the instructions things is kind of amazing!

 

Beth

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Sorry for your plaster problems. I have a terrible time with the dang stuff also - yes, I follow the recipes and read all I can about it. Last project was sort of a dud, but to get it to dry I put it in the wood stove oven for a couple of days. (I had given up on it , so what the heck?!) The clay tile did finally come out and the massive lump of plaster did dry!! Amazing. I have a lot to learn also, (typically the hard way) but the prolonged heat didn't hurt - lesson #1. ;-)

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