Marian65 Posted November 16, 2011 Report Share Posted November 16, 2011 I've never used plaster. I want to make a hump mold from an aluminum pan. My purpose is to make some serving dishes that will be the same size and shape. Can I use plaster of Paris? If so, what should I use for a release agent? I have a 50# bag of potter's plaster that is unopened and tightly wrapped in heavy mil plastic because I'm insecure about opening it and storing it without its gathering humidity and being wasted. I have some cartons of plaster from a hobby store, so I would prefer to use that for this little project if it will work. Marian Link to comment Share on other sites More sharing options...
Marcia Selsor Posted November 17, 2011 Report Share Posted November 17, 2011 I keep 50 pound bags of potters plaster in a large plastic bag in a large rubber storage container. I live is the tropics of SOuth Texas and it stays good for a year or more. I would not use Plaster of Paris for a mold for clay. IMHO it is too soft. Marcia Link to comment Share on other sites More sharing options...
bciskepottery Posted November 17, 2011 Report Share Posted November 17, 2011 For a release agent, I use Murphy's Oil. Link to comment Share on other sites More sharing options...
Marian65 Posted November 17, 2011 Author Report Share Posted November 17, 2011 I keep 50 pound bags of potters plaster in a large plastic bag in a large rubber storage container. I live is the tropics of SOuth Texas and it stays good for a year or more. I would not use Plaster of Paris for a mold for clay. IMHO it is too soft. Marcia Thank you, Marcia. I appreciate your response. I will breathe in some courage and get a container for the potter's plaster and make myself start using it. It certainly isn't doing any good taking up space in my studio while I continue to be wimpy about trying it. Some things I jump right into, but I've been shy about trying mold making for YEARS! Link to comment Share on other sites More sharing options...
Marian65 Posted November 17, 2011 Author Report Share Posted November 17, 2011 For a release agent, I use Murphy's Oil. Thanks for the suggestion. I have plenty of that. I wasn't sure whether Vaseline would be OK with plaster of Paris in aluminum, but now that I've changed my mind (see Marcia's note) about using that kind of plaster, I'll be courageous next week and make my molds with potter's plaster. Who knows; I might get good at it! :-) Link to comment Share on other sites More sharing options...
Lulu Posted November 18, 2011 Report Share Posted November 18, 2011 I use rainex as a mold release. I also don't fill the mold all the way up and then turn it over onto a folded towel and gravity does a great job of the rest of it. It falls right out! Just don't leave it in the mold more than 1/2 an hour- 45 minutes. Link to comment Share on other sites More sharing options...
Marian65 Posted November 19, 2011 Author Report Share Posted November 19, 2011 I use rainex as a mold release. I also don't fill the mold all the way up and then turn it over onto a folded towel and gravity does a great job of the rest of it. It falls right out! Just don't leave it in the mold more than 1/2 an hour- 45 minutes. Now how in the world did you discover that Rainex could be used as a mold release? I really enjoy using things for other purposes from which they were intended. How clever. Does it in any way affect the plaster after it's dry? Thanks! Link to comment Share on other sites More sharing options...
Marcia Selsor Posted November 19, 2011 Report Share Posted November 19, 2011 I use WD-40 as a mold release when casting plaster molds on found objects like wooden forms. Also have used Murphy's soap and vaseline. Never tried Rainex. Marcia Link to comment Share on other sites More sharing options...
clay lover Posted November 19, 2011 Report Share Posted November 19, 2011 I, too, confess to having a bag of potter's plaster sitting in my studio, looking at me, daring me to open it. I've read such mathmatically complicated articles about formulas for measuring and weighing to mix it. Is there a simple way to determine how much dry plaster to start with for a given form? Link to comment Share on other sites More sharing options...
Marcia Selsor Posted November 20, 2011 Report Share Posted November 20, 2011 If you mix in a one gallon bucket..here is an example of how I was taught to mix #1 pottery plaster. I have done so for years. Put water 1/3 of the bucket. take a scoop and slowly sprinkle plaster into the water, try to put it evenly around the edges , in the middle etc. Eventually there will be an island forming in the middle. I try to get my island to cover more than half of the top surface in the bucket. Let it stand for a few minutes. You can start stirring or you can use a jiffy mixer to slowly stir it ( variable speed drill). Let it rest. run a finger across the top surface. When the ripple doesn't go away, pour the plaster into your mold, form or whatever. I usually have several things standing by to use the excess plaster. Never put plaster down the sink. Scrape the excess out of the bucket into a garbage can. With practice you can estimate volume pretty well. Marcia Link to comment Share on other sites More sharing options...
BeckyH Posted November 21, 2011 Report Share Posted November 21, 2011 To estimate volume you will need to do some math. Set your cottle boards around the thing you wish to make a mold of. For slip casting I like a 2-3" wall around my object, and 2-3" above and below it. If you are making a one piece mold, you only need the volume above the object. When the cottle boards are where you want the, measure the size of the box you have made. Multiply that by the depth you need the plaster to be. If the box is 10"x 10"' and you need a total of 4" of depth, you need 400 cubic inches of plaster slurry. There are 80 ci in a quart. Multiply the dimensions and divide the total by 80. This will tell you how many quarts of water you need. 400 cubic inches / 80 cubic inches per quart water = 5 quarts A quart of water weighs 2# The proportion of plaster to water is- 2.75# plaster per quart For 5 quarts you need 13.75 pounds of plaster. There is no way to do this without some math, sorry. This gives you a 70% hydration of you plaster. Good luck! Link to comment Share on other sites More sharing options...
Marcia Selsor Posted November 21, 2011 Report Share Posted November 21, 2011 I have been using plaster for over forty years. I have to confess to never doing the math. I am very good at estimating volume. Just practice, I guess. I know there are many types of plaster and specific recipes for them. I use #1 pottery plaster. It has worked for me for decades. For people just starting out, U.S. Gypsum (http://plaster.com/ ) does have all the recipes on their website and directions for mixing various types of plasters. Marcia Link to comment Share on other sites More sharing options...
Idaho Potter Posted November 23, 2011 Report Share Posted November 23, 2011 As I've said before on other forums, industrial plaster (available at most building supply stores) gives a harder surface than pottery plaster and still gives good detail if the initial coating is splattered on the object to be cast. Whatever plaster you use, the biggest problem is undercuts. Undercuts are usually the reason molds are made in three or more pieces. Link to comment Share on other sites More sharing options...
clayshapes Posted November 23, 2011 Report Share Posted November 23, 2011 If I want to make a molds of a winter squashes -- is there any reason why I can't mix the correct amounts of plaster/water in a shallow plastic washing up bowl from the dollar store, and set the squash (rubbed first with vaseline) in it, and let it set? I see so many complicated instructions for using wooden boards etc. Is there a reason not to just use a plastic tub of the correct size (with a release agent rubbed/sprayed on first)? Link to comment Share on other sites More sharing options...
Idaho Potter Posted November 23, 2011 Report Share Posted November 23, 2011 Clayshapes, First you have to establish a parting line on the squash. That's not always a straight line. I caution you on finding the correct line for marking half the squash. If the parting line is the least bit off and you have problems removing the squash, your clay casting will not come out easily or at all. Go to your library and find books on plaster molds and/or model making. It will make your job easier. I have a book, Plaster Mold and Model Making by Chaney & Skee (ISBN 0-442-21515-0) but it may no longer be in print. Almost any book on the subjects will show you how to discern the parting line to get a two-part mold or show that you need more parts. Using a plastic basin with a release agent is no problem if you're willing to sacrifice the basin. As plaster cures, it expands and you need some way of releasing the restriction or ruin the mold. You can use scraps of linoleum rather than wood, but any coddle (form to hold the plaster) needs to be movable as the plaster cures. Really, get a book, there's too much specific information about making molds than can be covered in a couple of paragraphs (books also show you pictures of the process). Link to comment Share on other sites More sharing options...
Benhim Posted November 24, 2011 Report Share Posted November 24, 2011 I use Vaseline. It releases really well, but the thickness can create a problem with surface texture of the mold. To get by this problem I've used a heat gun to melt the Vaseline on the surface of the object so that it's thinner and smooth. I like the murphy's oil thing, I'll have to try it. Link to comment Share on other sites More sharing options...
DAY Posted November 24, 2011 Report Share Posted November 24, 2011 Idaho Potter is correct- only, "industrial plaster" covers many types. Ask for Gauging Plaster. It comes in 'regular' and quick set. Much, much cheaper than the stuff sold at ceramic stores. You won't find it at the Big Box places; go to a "real" lumberyard. Link to comment Share on other sites More sharing options...
Marcia Selsor Posted November 25, 2011 Report Share Posted November 25, 2011 I just read the Gauging Plaster page from US Gypsum. It is primarily a surface finish. You say it is good for molds. That is a nice thing to know. The set up time is listed below. I have to get #1pottery plaster from 280 miles away. If I could use this, it would probably be easier to access in deep south Texas. STRUCTO-GAUGE® Gauging Plaster mixes with lime putty to produce a high-strength, durable, white, smooth trowel finish. Its excellent hardness- and abrasion-resistance make it ideally suited for high-traffic areas and other applications that require durability and abuse resistance. The plaster should be used over high-strength, wood fiber or sanded, gypsum basecoats. It applies faster and easier than RED TOP® Keene's Cement. The plaster is available in two formulations: Quick Set formula sets in 30-40 minutes, making it ideal for use over low-suction veneer basecoats. Slow Set formula sets in 60-75 minutes, making it suitable for use over conventional sanded basecoats. The plaster should not be used with lightweight aggregates, portland cement basecoats, or masonry. It meets ASTM C28. Link to comment Share on other sites More sharing options...
~janie Posted November 25, 2011 Report Share Posted November 25, 2011 I have the same problem of having to get my plaster from a distance. If this plaster is available at our local lumberyard, it would save me a lot of time and MONEY! I think I paid $51.00 in shipping cost the last time I ordered,(and even then, UPS screwed up the order, said it was ruined, and I didn't get it for another week-grrrrr) and I have been reluctant to order any more. I am very stingy with it! Great tip! Thanks! Link to comment Share on other sites More sharing options...
Marcia Selsor Posted November 25, 2011 Report Share Posted November 25, 2011 Idaho Potter is correct- only, "industrial plaster" covers many types. Ask for Gauging Plaster. It comes in 'regular' and quick set. Much, much cheaper than the stuff sold at ceramic stores. You won't find it at the Big Box places; go to a "real" lumberyard. Is this as absorbent as pottery plaster? And can you cast it? Can you use it as a wedging surface? Marcia Link to comment Share on other sites More sharing options...
DAY Posted November 25, 2011 Report Share Posted November 25, 2011 Idaho Potter is correct- only, "industrial plaster" covers many types. Ask for Gauging Plaster. It comes in 'regular' and quick set. Much, much cheaper than the stuff sold at ceramic stores. You won't find it at the Big Box places; go to a "real" lumberyard. Is this as absorbent as pottery plaster? And can you cast it? Can you use it as a wedging surface? Marcia Yes, it works as a wedging surface- and in that use it is quite absorbent, when drying out wet slip. Note, also, in addition to being cheap, it is "fresh", if purchased from a supply house that sells a lot of it. "Pottery plaster" -indeed,ANY plaster- can go bad over time, as it is hydroscopic. Hence the advice to double bag it, and then store in trash cans. Link to comment Share on other sites More sharing options...
Guest The Unknown Craftsman Posted November 25, 2011 Report Share Posted November 25, 2011 Hygroscopic. Link to comment Share on other sites More sharing options...
arnold2benton Posted November 26, 2011 Report Share Posted November 26, 2011 You can pour plaster into another plaster mold to make the reverse of it. You need mold release in this case, or the plaster will stick. Link to comment Share on other sites More sharing options...
Marcia Selsor Posted November 26, 2011 Report Share Posted November 26, 2011 Having used #1 pottery plaster, casting plaster and hydrocal over the past 4 decades, I wonder how this gauging plaster compares to those for making molds, pouring, etc. I never heard the term before the recent post. I know Venetian plaster which is for decorative surfaces on sheetrock. The Gauging plaster description by USG seems more similar to Venetian plaster than a casting plaster. Can someone give more details based on their experience? Marcia Link to comment Share on other sites More sharing options...
Christine Posted November 27, 2011 Report Share Posted November 27, 2011 Clayshapes, I have a book, Plaster Mold and Model Making by Chaney & Skee (ISBN 0-442-21515-0) but it may no longer be in print. Almost any book on the subjects will show you how to discern the parting line to get a two-part mold or show that you need more parts. Some years ago I was gifted "The Essential Guide to Mold Making and Slip Casting" by Andrew Martin ISBN 978-1-60059-077-1 which I've found useful to dip into .... it's very comprehensive and contains tables for mixing ratios and as well as recipes for casting slip Christine Link to comment Share on other sites More sharing options...
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