Sanguine Posted November 28, 2021 Report Share Posted November 28, 2021 Is there a good way to determine how much plaster to mix for frosting a rounded somewhat half-cylinder form? Since I am not pouring it into anything, I can't measure volume. Link to comment Share on other sites More sharing options...
Chilly Posted November 29, 2021 Report Share Posted November 29, 2021 Find a bowl bigger than your master, but approximately the size the frosted mould will end up. Put bowl into sink. Fill bowl with water, right to the top, and submerge master into water. Measure the water left in the bowl. That will give you an idea of the volume required. Link to comment Share on other sites More sharing options...
PeterH Posted November 29, 2021 Report Share Posted November 29, 2021 @chilliYou explained the procedure a lot more clearly than I could have. But shouldn't you be measuring the amount of water left in the smaller bowl? Link to comment Share on other sites More sharing options...
Sanguine Posted November 29, 2021 Author Report Share Posted November 29, 2021 Eureka! How could I have forgotten about Archimedes? This will get me closer for sure. I think though that Peter is right. I don't want to measure the total volume of the piece, just the outside. Appreciate the help, Chilly and Peter! Link to comment Share on other sites More sharing options...
Chilly Posted November 30, 2021 Report Share Posted November 30, 2021 15 hours ago, PeterH said: But shouldn't you be measuring the amount of water left in the smaller bowl? oops, yes, I was getting over excited at being the first to answer ! thank you @PeterH. For clarity and future readers of this thread, I'll correct my error. Link to comment Share on other sites More sharing options...
JohnnyK Posted November 30, 2021 Report Share Posted November 30, 2021 Unless I'm wrong here, you should be measuring the water left in the larger bowl since that would represent the amount of plaster on the outside of the master form...Something to consider here is that if you use a "frosting" method to create the mold, any pieces that you make from the mold will be smaller than the master by the shrinkage rate of the slip or clay (if you make it a press or slump mold. Link to comment Share on other sites More sharing options...
PeterH Posted November 30, 2021 Report Share Posted November 30, 2021 I think it's less potentially confusing if we use the terms master, bowl & bucket. Initially the bowl holds the (estimated) volume of the mould plus that of the master. Putting the master into the bowl displaces water into the bucket, who's volume is that of the master. Take the master out, and then: - the water in bucket has the same volume as the master - the water in the bowl has the same volume as the mould Link to comment Share on other sites More sharing options...
Jeff Longtin Posted November 30, 2021 Report Share Posted November 30, 2021 A method I employ is to measure your model and calculate its volume. To simply the process run the numbers based upon a WHOLE form and THEN divide that in two to get your volume. Knowing that your mold will likely be 1.25" larger, all around, run the same calculations but 2.5" larger overall. Then subtract one from the other and you have the volume of the mold. To simplify the mixing process I use plastic storage containers to measure out my plaster. I then use a graduated cylinder, an old Coke bottle, to measure out the water. A 32 oz plastic container contains the equivalent of 38 sq in of wet plaster. (This requires 15.5 oz of water.) After I run the numbers I divide the number of square inches by 38 and determine how many cups of plaster I need to measure out. I employ this method so I'm not mixing an excessive amount of plaster on each pour. Link to comment Share on other sites More sharing options...
Sanguine Posted November 30, 2021 Author Report Share Posted November 30, 2021 Thanks everyone for all your input! I think I will attempt it using the water displacement method and measure the water left in the outer container originally described and clarified by @Chillyand @PeterH I am going to mull over @Jeff Longtin's method as I am having trouble getting it all to make sense. I think I need to run some numbers and perhaps even do a trial run with a smaller and easier piece just to see how the measurements work together and what happens in the end. One of the problems is that this is not a simple shape, but a series of graduated cylinder shapes that are arranged side by side from smaller to bigger to smaller again to create a cloud shape. I would rather mix too much than not enough - I can always use extra to make more hump molds. And thanks for the reminder about shrinkage, @JohnnyK - it's always a shock to see how much things shrink! Link to comment Share on other sites More sharing options...
Sanguine Posted November 30, 2021 Author Report Share Posted November 30, 2021 Oops: graduated HALF cylinder shapes. The cloud shape widens out as it goes down and is flat on the bottom so will work as a one-piece mold with no undercuts. Link to comment Share on other sites More sharing options...
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