another plaster question

[skeets52843]

I want to build a 3'x3' open cube and fill it with plaster. I then want to carve on it and chip away at it. Basically, I want to do the same project everyone did when they were in kindergarten with the milk cartoons, but on a MUCH bigger scale. I am worried that the plaster will not dry out all the way though. Does anyone know if this will work? Does anyone have any suggestions for anything else? Thank you in advance for all of your time.

[jwebb]

Have done this many times, using ordinary cardboard boxes and it's quite easy. Suggest you mix the plaster in a rubberized or plastic pan, starting out with about half a pan of water and putting plaster into it by handfuls, sifting it in, until islands form which no longer disappear. Mix it up, squeeze out any lumps, until you have a nice creamy consistency, and pour it in the box. Have any seams taped up first. It'll set up in about 30 minutes or so. (It is exothermic when it sets up, i.e., it gets warm). Then just mix up another pan and pour it on top, and keep it up until your box is full. 3' x 3' is a bit larger than I've done, but not much. The water doesn't really "dry out" in plaster, it goes into hydration. Same with concrete. If you just want to carve the plaster to produce a finished piece, you can enhance it by adding (before you pour it) some color, and/or a handful or two of vermiculite as a conglomerate. IThe plaster will look rather like granite, with speckles in it. Wood carving chisels work fine on it. "Sureform" tools and rasps have a lovely effect. I've also made plaster carvings this way, then made a piece mold over it (also plaster, built up over a mold release of liquid soap), and cast a finished piece inside that mold, in concrete. Have also seen this done and a "bondo" or fibreglass final piece built up inside that piece mold, which is then chiseled off. Hope this helps. Let us know how it goes. Here's a lousey photo of a concrete piece, done as above.

[fritchie]

Here's something to think about. If you’re talking about 3 feet deep as well as 3 x 3 ft. in cross-section, that is 27 cubic feet of plaster. I think water is about 63 pounds/ft.3 (???), so that much water would weigh about 1700 pounds. Plaster probably would weigh closer to 3000 pounds or so (???).

You should look into a more accurate estimate of weight. A trick that should help, if you proceed with any such large casting: Make your box with removable sides.

[anne (bxl)]

also, skeets, working such a quantity in once is not kindergarden anymore. to have a creamy substance without lumps you need to use a professional mechanical mixer. and remember, 1cm (let's say half an inch) deep needs about 2 to 3 weeks to complete the hydration process.

[jwebb]

True statements by Anne and Fritchie. Make this block where you want it to be while you carve it, and/or have some means of moving it afterwards. However, you don't need the block to be as dry as cuttlebone to start carving it. That might apply in some applications, but not for making a block to carve. And don't worry, it will complete the hydration process long before you're done carving! The key, I think, is to get a nice, creamy mix. Probably use less than 1/2 a pan of water, use 1/3 of a pan, and add plaster to water not the reverse. The plaster soaks up the water in the mixing, and if it's a nice consistency at that point, there will be no excess water after it sets.

[DirtEater]

I know this is an OLD thread, but I have a question:

Doesn't plaster, even Hydrocal, shrink at all? Wouldn't a solid 3'x3'x3' cube crack a lot because of even slight shrinkage?

[Landseer]

I know this is an OLD thread, but I have a question:

Doesn't plaster, even Hydrocal, shrink at all? Wouldn't a solid 3'x3'x3' cube crack a lot because of even slight shrinkage?Plaster and hydrocal EXPAND slightly, the one problem I have with pouring plaster in a form and then mixing up another batch and pouring that in on top of already setting plaster is THIS can cause cracking, hard spots and other anomalies in the fresh layers.
Why? think about it, the first pailfull is dumped in the form, now you go get another bucket of water, add plaster to it, meanwhile that first batch in your form is already setting up and is starting to get warm. At that stage this plaster will absorb water like you can't believe- you go and dump a fresh batch of plaster on top of this and it starts sucking the water out of the new batch "Shorting" it so the correct ratio of water to plaster has been changed. Meanwhile this water being sucked into the already setting plaster layer which is now getting HOT is probably not good for that.

The one already setting layer is starting to expand and heat up, the fresh layer is starting to set and now is being stressed by the expansion of the previous layer which is expanding while the fresh ISNT yet, this can cause cracks in the still soft but barely setting layer.

Lastly, different batches plus the above will result in odd alternating hard and soft spots which can cause issues with carving.
On my hydrocal casts if I run short in a pour I NEVER add more to the pail or the mold. The exception is casting a large piece, in which case I have TWO 5 gallon pails already full of water, add plaster to both, drill mix one, mix the other, and then mix the two together using a third emplty bucket THEN pour in the mold.
That way I have 10 gallons and all is the same ratio and mixed together.

A bit much for 27 cu ft but you get the idea.

Hydrocal etc weighs about 110# a cubic foot as I remember, concrete about 150, plaster maybe 100.

I disagree with the statement that plaster doesn't dry out but hydrates, it DOES hydrate but it ALSO dries out all the excess water.

USG literature suggests a heated dryer with fans to DRY OUT plaster casts once hard. There is still a considerable amount of free water that must dry out and 3 foot cube will eventually dry out from the inside out, but it could take weeks. You would want to carve it wet anyway so that's good.

Near as I can tell, approximately HALF of the water you add to the powder STAYS there- it's chemically combined, the other half dries out.

I discovered the aproximate 50% when I took several large casts I knew exactly how much hydrocal I used for and how much water I used, and anticipated their end weight would be the total weight minus the water but discovered that was not the case, that about half of the 5 gallons of water I used stayed there even though the cast was fully dry weeks later.
In other words, say I used 60# of hydrocal and 5 gals of water (40#) that adds up to 100#, I expected the cast would dry and wind up 60# but it was 80# which meant only half the water evaporated.
I hadn't thought of that before but after that it made perfect sense when you know plaster made by burning off all the water and hydrates to harden up- taking the water back.

However much plaster you use plus water, figure your raw cube would weigh the amount of the plaster plus 50% of the weight of the water used and that would be approximate DRY weight.

Im not sure if USG's weight per cu ft charts accounts for this or not, it may include this.

Lastly, you are talking about mixing up a few thousand pounds of materials here, you MUST do this with more than a "pan" or measuring cups, you will want at least several plastic 5 gallon buckets, an electric drill and a large jiffy mixer for the drill. Even so, for 27 cubic feet you are looking at about 8 gallons per cubic foot x 27= 216 gallons, or about 40 of those pails, and about 2 or 2-1/2 gallons of each 5 gallon pail is water.


If you can get half a dozen of them and fill them all with water, and have a helper it would go more or less continuously.

You might have less headaches building a good solid form and having a truck fill it with sand mix concrete -no gravel or rock.