I picked up a few WS rock molds tonight and a 25# bag of Plaster of Paris today and want to do some rock castings. I mixed up my first batch but it went thick on me in a hurry. So I’m looking for some mixing help.
The bag has no suggestions. I’ve heard that I want it about like pancake batter but it went from thin gravy to mustard to thick pancake batter to mashed potatoes in about two minutes. Didn’t do something right. Can someone give me some mixing instructions by volume for mixing up a batch that is good to work with?
Thanks.
dlm
I’ve had good luck mixing plaster of paris in a 1:1 mixture with water. I use a typical red plastic drinking cup that you get in a sleeve of 50 at Wal-mart. But it does start setting quickly so your molds have to be ready. Here’s the deal, from what I understand. You can’t mix paris of plaster in different ratios. It has to be 1:1, if you try to add more water, it will not setup correctly and will be weak later. Weak, as in crumbly. I do not hold the molds in place, but let the molds set for 20 minutes. I then remove the castings from the molds and let them set for 24 hours, then set them into place later with plaster. My plastic red cup usually fills two WS molds. Good luck!
Keith Baker
I used the DAPP or whatever it was Plaster of Paris in the two quart milk-type carton and followed the instructions. I seem to recall, now four years hence, mixing about a cup of the plaster and maybe 1/4 cup of water, but I doubt my memory. Whatever they said, it worked and I poured it directly into my molds as soon as I could see that there was no dry powder. I lightly lifted and dropped the mold a few times to ensure proper settling and seating of the plaster slurry and then walked away for about an hour or so. I have read, since, that some folks use vinegar as a retardant. It adds perhaps 20-30% to the apparent set-up time.
I found out that if my slurry began to stiffen as I was mixing it, I just added another tablespoon of water and kept mixing for another few seconds, maybe 5 at most. If it wanted to thicken again, I added still more water. Eventually I could pour a well-mixed slurry into the mold and the rest was as I described above. I never broke any castings once they were removed from the mold.
-Crandell
I read in Bill McClanahan’s scenery book that you can buy yourself some time by adding a little vinegar to the water. I no longer have the book to look it up and get the amount of vinegar per unit of water, so you’ll need to experiment. I never tried it myself, it’s just something I read in an old (but very good) book.
Hey Dan, welcome to the “the darn plaster set up before I got it all poured into the rock molds” club. I used Hydro-cal lite, but had the same problem, at first. I set up my molds on a flat surface with pieces of wood to get them level, then lightly sprayed them with “wet water”. I used a 1 qt. Pyrex measuring bowl with a spout on the side. I put the water in and added the plaster, and mixed with a ss wire wisk. It has to be mixed fast. Then I poured the mix into the molds and used a rubber spatula to get all the mix into the molds. Hope this helps. DJ
Start with a cup of water,slowly add plaster til it has absorbed all the water, use your fingers to mix the plaster a little so you can feel the lumps and mix them in.Wear rubber gloves if your squeamish but very few people are affected adversly by the wet mix. Try not to breath the dust though. Pour your plaster into the mold. Watch for the initial set then gently mash the mold against your plaster base, Alternate, use a pallette knife to apply the plaster to the base modeling it as you go. You WILL waste some plaster getting this to work. DO NOT put plaster down the sink or toilet. It will set up in the traps. Get a 5 gal bucket half full of water to clean your tools and bowl in. I dispose of it out side in the garden or a gravel area. Looks funny for awhile then disappears. If you let the bucket set a few days the plaster will set up, then you can pour the water off and dispose of the plaster in the trash. You can color the plaster with RIT dye but it will probably fade after a year or so. Best to use acrylics after everything is set.
Dan
I also prefer Hydrocal light as if you use plaster or paris the finished product isn’t much lighter then the real thing. After trial and error I have come up with a pretty fool proof method. I made small wooden boxes just big enough to hold the ends of the mold sort of like a frame to hold it up off the table. So this way the mold isn not deformed. with the larger molds I put them in box lids and filled them with either crumpled up newpaper or anything I had laying around to support the mold but not deform it. I finally found a use for those damded packing peanuts no kidding. I followed the direction on the box to the letter but found it pours a little better if you add a tad bit more water. Yes you have to liberally spray the mold with wet water before hand so it acts as a release agent.
A little tip on painting them, go to Woodland Scenices website and check out the little short tutorial on painting rock castings. At first I thought their choices of colr was off the wall but the first patch I painted came out perfect.
I recently did some WS molds with plaster of Paris. I found the plaster worked best (more predictably) when fresh COOL (or cold) water was used. Didn’t think something that simple would make a difference, but it did.
I also made sure the molds were already soaked with soapy water. They released easier once the plaster set up.
For rock molds I make a thin mix, first water then plaster. And stirring! If it gets to thick I add more water.
Wolfgang
I prefer to use white vinigar as a retardent. It makes a big differance in setting time, but doesn’t affect the casting.
Cold water helps, to retard setitng as well, the warmer the water the quicker will be the setting time. The chemical reaction causes heat.
Yes this is very true. You do not want to use hot water but use cold water if you want it to take it time.
Also I have heard that baking powder can help slow down the process.
Personally I use premixed drywall mud that I apply right on the scene then once its mostly dry I start carving my rock formations. I believe it turns out just as good as castings do.
First, thanks to all of you who replied to my request. Through some trial & error, I have found that two parts plaster of Paris by DAP and one part water by volume works pretty well. But I did find that once the plaster mix started to go, it went fast in terms of setting up. So pour it before you think you really need to and it is much easier to work with.
One of the things I did was to make a cardboard ring around the base of the mold on the back side then poured plaster into the ring after spraying the back of the mold with wet water. When it set, I had a very good and level support for the mold to work with.
This mix may not work for everyone as it appears that different types of plaster take different mixes. Still don’t know why they couldn’t have printed that on the bag or box. : (
Thanks again!
dlm
Borax will slow down the plaster setting & salt will speed up the plaster setting. I bought each of the WS molds and cast each rock. Then I coated the castings with a release agent and made my own molds with latex rubber, using cheesecloth at 45 degrees. The latex molds work easily on curved surfaces whereas the WS molds do not. You can support the latex molds in cat gravel when you pour the plaster in the mold. Bob Whitten
the instructions from the plaster paris I got from Lowes said 3 cus of plaster paris per cup of water. I just added the cold water to plaster paris, mixed throughly, added a bit more water to get to thin pancake like mix, and poured into WS molds that had been misted with wet water.
its drying now
the cold water retards the exothermic reaction. after pouring six molds I was able to clean the tin can out with fresh water without any problem.
Bill D
1/2 tsp of white vinegar per cup of mixed plaster will retard the setting sufficiently to allow you to mix and pour.
As someone mentioned above, I set my molds in a kitty litter box filled with sand to keep them level while they set. A word of advice – COVER this box with something heavy, even if it’s sitting up on your workbench if you have a real cat in the house. Don’t ask how I know this! [B)]
I have always mixed plaster of paris 2 to 1. 1 cup plaster to 1/2 cup water. It must be used quickly as it starts to set instantly. Mixing it 1 to 1 makes it too thin and therefore weak in my opinion.
Yes – 2:1 is what I use, too.
DLM … I’m not even going to go any further down the posts, although I’m sure there is good advice – I’ll answer directly. In design school I had a freshman class called Plaster Workshop. It was all about making patterns and then molds from the patterns and then a production run from the molds. At any rate, here’s the drill.
Start with small batches till you get the hang of it, say two or three cups of water in a plastic container (a cut-down 1/2 gal. milk container washed out thoroughly works.) Add the plaster to the water, sifting it in through your fingers (or a sieve if you’re as anal as I am) until there is a very small mound of what looks like dry powder (it isn’t, but you can kinda see granules) in the middle. O.K. You’ve already discovered that plaster sets up in a skinny minute, so … stir with your hand, “become one with the material,” and you will know when the mix is getting ready to set by the increase in temperature – it starts getting warm, and then hot. Pour quickly before “hot.”
As far as exact proportions are concerned, I 've heard 2pts plaster to 1pt water but I’ve never measured … I’ve always just used the method I just described and it has never failed. When you’re finished, if you can’t knock out any dried palster left in the cut down milk jug, toss it in the recycle bin and cut down another one.
Hope that was helpful. This method has worked like a charm for me since 1966. It’s a winner!
biL
1/2 teaspoon of white vinegar per cup of plaster will retard setup time by about 30-50%. The mix ratio of POP to water is also affected by humidity, so you have to get a feel for the mix in your environment.