Big Ray Posted May 14, 2015 Report Share Posted May 14, 2015 What's up everyone. I was wondering if any of you guys bake your plaster of Paris molds. I usually just let them dry out on there own for at least a week. But the last one I made I wanted to try baking it. It came out good but it took forever. I was baking at 350 degrees and I would check it every 30 min but every time I took it out of the oven it was steaming. I did that for a good 3-4 times so it took a while. Then I got fed up and just raised the heat to 375 for another 30 min and it finally seemed like it was dry. I was just wondering what is the best temperature to get it done fast. Quote Link to comment Share on other sites More sharing options...
Vodkaman Posted May 14, 2015 Report Share Posted May 14, 2015 The problem with plaster is that there is a temperature beyond which a chemical reaction takes place and the mold turns back to the original stuff before you added water in the first place. When oven drying, as long as there is water still in the mold, it cannot hit that magic temperature, held back to boiling point until all the water has been driven off. Once the water is gone, the mold temp rises with the oven and unless you rescue the mold very soon, it will be ruined. I did some experimenting, the results might be of use to you: I weighed the freshly made mold and weighed the fully cured mold. The weight loss was 30% every time. I also discovered that the rate of weight loss was constant. This means that you can weigh the mold after a few hours (air drying or I use a light bulb box heater), you can actually predict when the mold will be dry. You could apply this method to oven drying to get an idea how much water is left. The water loss is not likely to be constant, so I would aim for 27% and call it a day. In a normal living room, you are looking at a week. Other ideas are, radiators, airing cupboards, the back of a refrigerator is a good one. With my box, with 2 x 100w bulbs and a computer fan to keep the air moving, I could dry a mold in 6 - 8 hours. I would leave it on over night because you cannot over dry a mold provided you stay below the magic temperature. Dave 1 Quote Link to comment Share on other sites More sharing options...
Big Ray Posted May 14, 2015 Author Report Share Posted May 14, 2015 Wow Vodkaman. I read some of your other posts and I noticed that you get really scientific. So I guess I need to get myself a good scale. This information is going to come in handy thanks. Quote Link to comment Share on other sites More sharing options...
Vodkaman Posted May 14, 2015 Report Share Posted May 14, 2015 It was a successful series of experiments and a lot more to it than I have written here. I even built an electronic moisture sensor that started beeping faster and faster as the lure dried. That was fun, but not recommended if you're married Actually, ordinary kitchen scales will do for this job. Dave Quote Link to comment Share on other sites More sharing options...
JRammit Posted May 15, 2015 Report Share Posted May 15, 2015 Ray, i put mine in for 20 minutes on warm (150-170).. Seems to work, at least they are dryer than when i put them in... Dave, what are u using plasrer for?... Didnt know u made soft baits??? Quote Link to comment Share on other sites More sharing options...
Vodkaman Posted May 15, 2015 Report Share Posted May 15, 2015 I have made quite a few different things, not all to do with fishing lures. I made a lot of resin bodies for testing hunting lures. I did a lot of molds when I was developing the vacuum molding method. When I did the plaster experiments, I was not making anything, just curing blocks of plaster. Dave Quote Link to comment Share on other sites More sharing options...
JRammit Posted May 16, 2015 Report Share Posted May 16, 2015 I have made quite a few different things, not all to do with fishing lures. I made a lot of resin bodies for testing hunting lures. I did a lot of molds when I was developing the vacuum molding method. When I did the plaster experiments, I was not making anything, just curing blocks of plaster. Dave Dave... U got any useful links pertaining to this "vacuum molding"???... I had before pondered somehow connecting my little 110v pump for inflating and deflating pool toys to the flush vent on one of my molds, but have yet to try it... Was i on the right track? Quote Link to comment Share on other sites More sharing options...
Vodkaman Posted May 16, 2015 Report Share Posted May 16, 2015 (edited) JR - I just added missing images to two posts on the subject of vacuum venting. Very early work so be nice. Hairy worm 3D legs Let's not hijack this quality thread. Any comments on the old post please. Dave Edited May 16, 2015 by Vodkaman Quote Link to comment Share on other sites More sharing options...
Travis Posted May 17, 2015 Report Share Posted May 17, 2015 (edited) My non scientific response (from a scientist) no need to bake a mold in the first place. If time of of critical essence I just seal the mold and pour. Don't seal the back as it is going to sweat. Once done leave out on a rack to finish drying. Edited May 17, 2015 by Travis Quote Link to comment Share on other sites More sharing options...
JRammit Posted May 17, 2015 Report Share Posted May 17, 2015 Travis is correct... The mold is useable right after sealing, baked or unbaked, and will thoroughly dry over time... I only recently started baking mine due to a molding problem (the green kind).. I would use them a few times, put them away in my drawer, pull them out a month later and they were covered with fuzz..... Havnt had the issue since i started baking them Quote Link to comment Share on other sites More sharing options...
Big Ray Posted May 17, 2015 Author Report Share Posted May 17, 2015 Wow thanks Travis and JRammit. That is some good advice. That makes things so much simpler. Quote Link to comment Share on other sites More sharing options...
mark poulson Posted May 20, 2015 Report Share Posted May 20, 2015 I remember reading here, years ago, that 150-170 was the temp. to dry POP molds in the oven, with the door ajar. Quote Link to comment Share on other sites More sharing options...
SlowFISH Posted May 20, 2015 Report Share Posted May 20, 2015 I remember reading here, years ago, that 150-170 was the temp. to dry POP molds in the oven, with the door ajar. Key is in Mark's last 4 words - leave the door ajar.... the water in the mold has to evaporate. I have a little 14x14x14 scientific oven I use for things like this.... with the door sealed molds just gets hot and humid and don't really dry... leave the door open a little and it's 10x faster. You need that air circulation to "wick" the moisture away.... otherwise it's just a hot wet mold instead of a room temp wet mold. J. Quote Link to comment Share on other sites More sharing options...
Big Ray Posted May 20, 2015 Author Report Share Posted May 20, 2015 Thanks guys. No wonder it took so long to dry I left the door closed. Quote Link to comment Share on other sites More sharing options...
rixon529 Posted May 31, 2015 Report Share Posted May 31, 2015 The following is taken from USG (US Gypsum) web page entitled "Drying Plaster Casts": http://www.usg.com/content/dam/USG_Marketing_Communications/united_states/product_promotional_materials/finished_assets/drying-plaster-casts-application-en-IG502.pdf Quote. The main physical limitation in drying a plaster cast is the maximum temperature at which the dryer can operate and not calcine the cast. Recommended temperatures are 110 to 120 °F for USG® White Art Plaster, No. 1 Casting Plaster, Moulding Plaster, Pottery Plaster, HYDROCAL® Brand White and HYDRO-STONE® Gypsum Cements; from 125 to 130 °F for Industrial Plaster PC. Operating much above these temperatures will result in surface calcination; that is, surfaces of the casts, especially those in front of hot-air ducts, will become soft and powdery. Unquote. Quote Link to comment Share on other sites More sharing options...
Big Ray Posted June 3, 2015 Author Report Share Posted June 3, 2015 Thanks Rixon. I need to read this. Quote Link to comment Share on other sites More sharing options...
mark poulson Posted June 3, 2015 Report Share Posted June 3, 2015 (edited) Ray, remember, you're trying to dry the pop, not cure it. It cures as it dries and sets. It will air dry, if you leave it long enough. People put it in a oven or a drying box just to speed up the drying process. If I remember right, Vodkaman Dave actually weighed his pop before and after he added water, and then dried it until it reached a percentage of the dry pop weight, so he was sure of how much water was left. I'm sorry, but I don't remember what that percentage was. Edited June 3, 2015 by mark poulson Quote Link to comment Share on other sites More sharing options...
Vodkaman Posted June 3, 2015 Report Share Posted June 3, 2015 Mark - Yes, I covered it in post No2. The weight loss from a newly poured mold to a fully dried mold is 30%. What is worth mentioning here, is that there is no need to go for a totally dry mold, especially if oven drying. There are other indicators that show how wet or dry a mold is: Hold a glass near the mold removed from the oven. If the glass clouds up with heavy condensation then it is still wet. The surface of a dry mold feels just warm to the touch, even if it has just come out of a hot oven, it can still be touched and feels just warm. If you tap a dry mold with your finger nail, it has a 'ping' sound to it, a wet mold has a dull thud if you can hear anything at all. Keep a dry mold close by for making comparisons until you build up experience which will tell you everything. Dave 1 Quote Link to comment Share on other sites More sharing options...
mark poulson Posted June 4, 2015 Report Share Posted June 4, 2015 Dave, I guess I read it when you first posted it on this thread, but forgot. Doh!!! Do you ever get the moisture back down to the dry weight, or is some of the water locked up in the pop, like when some water is still locked in set and cured concrete? Quote Link to comment Share on other sites More sharing options...
Zunebuggy Posted September 12, 2020 Report Share Posted September 12, 2020 I was hoping I could bake plaster of paris at a high enough temp to cast a pine cone in it and then burn the pine cone up in an over outdoors and make a clear resin pine cone using the mold. I guess 375 won't do that. I saw another post that said 2200 degrees F was the max temp but that seems very high. Quote Link to comment Share on other sites More sharing options...
