evsarts Posted April 23, 2019 Report Share Posted April 23, 2019 Hi Folks, Pretty new to making plaster molds for slip casting. I have had some success but one recurring problem... sometimes when I take the item I am casting out of the mold, the plaster seems to tear, leaving an uneven surface. Many little bits of plaster are stuck to the original item, leaving lots of little divets in the mold. What is going on here? Am I demolding too quickly? Not mixing the plaster thoroughly enough? Mold release not working? I demold when the mold is warm to to the touch, about 40 min after pouring. I mix the plaster with a paint mixer on a drill, 2 min for 0-20lb, 4 min for more. I do not have a good system for sifting the plaster into the water, however. For mold release I brush on several coats of 50/50 Murphy's soap and let it dry. This has worked well for me, after many early failures with other methods. What I found in those failures though was very hard to remove the item from the mold, not that the plaster would fail in this way. Any insight appreciated! Link to comment Share on other sites More sharing options...
Mark C. Posted April 23, 2019 Report Share Posted April 23, 2019 Several ideas 1st and most important is your #1 pottery plaster new or old???It is #1 pottery plaster?? 2nd no need to seive it-which leads me to belive its lumpy and old . You should add it slowly to the water while mixing.-Two personsreally help here-shake it into the spining water in a larger bucket I use a jiffy mixer in a cordless drill. If its lumpy toss it out and get NEW plaster for molds-old plaster will be trouble alway the way along-Are your molds made from old plaster??? I'm not a fan of murpys oil soap-use lube made for mold relase which also will not clog the plaster pores this is made for that job https://www.axner.com/pure-lube.aspx I was in the slip cast business once and making quality molds that work is the whole deal. The right ingredients make the right molds 3rd Neil posted this in another thread (With #1 Pottery Plaster, you'll need 16.316 grams of plaster per cubic inch. Plaster to water ratio is 10:7 by weight.) this is the fomula you need to use always-weight the ingredients 4th I assume the plaster coming loose is in the working molds ???not the master? Master are made from rubber and will not shed plaster if thats true that the working molds are shedding sounds like molds where made with old plaster or are super wet when poured? to dry your molds blow air over them with a box fan any of this help?? let us know Link to comment Share on other sites More sharing options...
evsarts Posted April 23, 2019 Author Report Share Posted April 23, 2019 I’m not sure of the terminology but my post is about making the mold itself, not pouring slip into the mold. Most recently I was making a mold from a large turtle shell. I am using #1 pottery plaster and it is not old— the place I buy from seems to run out frequently so I assume it is fresh. I mix by weight, 7 parts water to 10 parts plaster. I can try adding more slowly... instructions I found are tight on time though. It does not seem lumpy when I pour it. Link to comment Share on other sites More sharing options...
Mark C. Posted April 23, 2019 Report Share Posted April 23, 2019 Ok Does the mold have some small undercuts which are catching the plaster? Turtle shells have under cuts-so are you just casting the top of shell with no undercuts. I would let the mold cool off before pulling it as its expanded when warm and will be harder to remove.Also the plaster needs time to get hard and its soft when warm still. I would still add the paster fast as its a time thing and you need to be fast. I drop the bucket on a firm surface after power mixing to raise any air bubbles to the top before pouring as to pouring start filling slowly so air does not get trapped in mold. Once covered our faster. Link to comment Share on other sites More sharing options...
Chilly Posted April 23, 2019 Report Share Posted April 23, 2019 +1 to everything @Mark C. has said. Another thing you could try, apply the first coat of plaster with a large soft brush, to make sure it is well covered, and no air bubbles. When pouring the plaster, make sure it does not hit and bounce off your model, but hits the baseboard or the side walls/cottles. I don't unmould until the plaster is cold. I do occasionally get a mould with tiny pinholes, but they don't affect the finished pot. Link to comment Share on other sites More sharing options...
CactusPots Posted April 23, 2019 Report Share Posted April 23, 2019 I think you need to take the first impression not with plaster. I was buying silicon products from Smooth On but found ammonia latex works better. You just build up 4-5 layers and then it peels right off. Then you get the plaster off the latex mold. Of course, now you're working with a negative, so you might have to go twice. I've been using this approach to take textures off some difficult rock specimens, like crystals and it really works. Link to comment Share on other sites More sharing options...
neilestrick Posted April 23, 2019 Report Share Posted April 23, 2019 Make sure you don't have bubbles in your mold release soap. Any bubbles will cause the plaster to be thin and set weird in those areas. Could also be that your soap is too thick. Link to comment Share on other sites More sharing options...
CactusPots Posted April 23, 2019 Report Share Posted April 23, 2019 I've been using Vaseline in turpentine for mold release. Got the tip from a mold master. Seems to be the ticket. Link to comment Share on other sites More sharing options...
1515art Posted April 23, 2019 Report Share Posted April 23, 2019 When making plaster casts of things I’ve had pretty good luck using olive oil as my mold release agent, the spray cooking olive oil works well. Link to comment Share on other sites More sharing options...
evsarts Posted April 24, 2019 Author Report Share Posted April 24, 2019 Thanks all for the adivice! I think I may be demolding too early, it seems like the problem is the plaster not setting well or not fully set. Somewhere I read that I should demold when the mold is warm, but perhaps that’s wrong, has anyone else done that? Could also be too much soap, thats a good thought. I have been avoiding oil-based mold release agents on the advice that the oils can clog the pores of the plaster making the mold less effective, but maybe it’s time to give those a try. Link to comment Share on other sites More sharing options...
Mark C. Posted April 24, 2019 Report Share Posted April 24, 2019 ( oils can clog the pores of the plaster making the mold less effective,) this can happen unless you use a non oil based release https://www.axner.com/pure-lube.aspx Link to comment Share on other sites More sharing options...
CactusPots Posted April 24, 2019 Report Share Posted April 24, 2019 As if using Stoner as a name for your business isn't recommendation enough. https://www.stonermolding.com/ Link to comment Share on other sites More sharing options...
1515art Posted April 24, 2019 Report Share Posted April 24, 2019 It probably works for me because I seldom (never) do slip cast objects and do mostly press molding with my plaster casts, I’m only interested in the quality of the mold surface and if the object releases from the mold, dryings not an issue and so far I’ve never had issues attaching parts or with the firings because of the oil. I guess I’ve used it for years because it good on the skin and was handy one time when I needed something not knowing any better, I’ll have to try the soap. Link to comment Share on other sites More sharing options...
Chilly Posted April 25, 2019 Report Share Posted April 25, 2019 Whatever you use for release, clean your new mould with vinegar, rinse well and allow to fully dry before casting. Link to comment Share on other sites More sharing options...
AspenLeafStudios Posted February 26, 2021 Report Share Posted February 26, 2021 I know this is an older thread, but I'm going to post now in case others see it later on. I had the exact same problems you did with plaster sticking to the molding surface (urethane rubber, silicone rubber, polyurethane resin, plaster, didn't matter) and breaking off in chunks. What I didn't see mentioned above, is what did the trick for me. Make sure your water you're mixing your plaster in is cold. The colder the better. I'm on mountain well water and it's downright biting when it comes out of the sink. This has solved everything. I use the Murphy's too, though I want to try the mold lube. Use the 10 plaster:7 water mix ratio. Get a scale (I have an Escali that I love - they come in all kinds of fun colors and can be found inexpensively on Amazon), and tar out your mixing bucket. However many ounces of water you have in your bucket, divide by .7 to figure out the ounces of plaster you need. i.e. if you have 25 oz of water, 25/.7=36 oz (rounding) of plaster. I've been hand-mixing - though I just got a mixer for my drill which I'm super excited about! - and I have flawless, bubble-free results in my plaster every time. Pour enough on to cover your surface detail, vibrate and then pour the rest. Good luck to all those playing with plaster! Link to comment Share on other sites More sharing options...
Bryan D Posted March 5, 2021 Report Share Posted March 5, 2021 Also I would add that if you're using mold soap or sealer that is mixed with water, make sure your model is 100% dry before pouring plaster. If your release agent is still wet, the plaster mold will have soft spots and can crumble/leave divots. Link to comment Share on other sites More sharing options...
PatrickEMT Posted June 28, 2021 Report Share Posted June 28, 2021 Hi Friends I had to remake a lid mold which broke to match another piece. I had a finished ceramic lid for a model. I found painting the model with latex ammonia and building up made a great form. I filled it with first plaster and then paraffin. The paraffin was perfect. I used the paraffin to make my new mother mold and when I slip cast it the lid was perfect yet it was smaller by 1/2 inch in diameter. any ideas? Link to comment Share on other sites More sharing options...
Mark C. Posted June 28, 2021 Report Share Posted June 28, 2021 Every time you use clay for a mold it shrinks the percentage of the clay or slip.hence the 1/2 inch Link to comment Share on other sites More sharing options...
PatrickEMT Posted June 29, 2021 Report Share Posted June 29, 2021 Yes I don’t know why I wasn’t thinking about that? I was so focused on the plaster that I guess I had tunnel vision! I realized this late last night so I guess I’m not totally mental!! thank you!! Link to comment Share on other sites More sharing options...
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