| This article was written by Christopher Pardell, Sculptor, in March 2000. |
| Plaster. |
| This remarkable material is stored as a powder, mixed with ordinary water into a liquid that gets gradually thicker and thicker until it becomes plastic, then a paste, then a cheese bodied mass and finally turning rock hard in about an hour. Unlike practically any other compound, when plaster turns from liquid to solid it does not shrink, rather, it EXPANDS ever so slightly as it forms a crystal. It can be carved, sanded, drilled, cut, textured, added to, reinforced and remains incredibly strong throughout. Because it expands, plaster castings do not lose any detail, and a mold with a glass smooth surface will result in a glass smooth plaster casting. |
| Plaster is hygroscopic, meaning that it absorbs water and can therefore be used to make molds for casting porcelain and ceramic slipware as well as latex forms such as those aliens and monsters used in the Star Wars movies. Plaster bats are used in the controlled drying of pottery and plaster is used to cover the walls in quality homes. Plaster can be "screeded" with a template to form straight and curved patterns with any cross sectional contour. At one time, virtually all decorative moldings in houses were of plaster and, of course, plaster is the principal material of pattern making and foundry processes. |
| In moldmaking, plaster is used to make your mold casings (mother molds), waste molds, casting and retouching masters. |
| Damn near all the plaster in this country is manufactured by U. S. Gypsum, (USG), And there are quite a few different kinds. Pottery plaster, number one casting plaster, art plaster, hydrostone, ultracal 30, ultracal 60, Moulding and so on. Each of these products have different characteristics that make them well suited to particular tasks. Some have surface hardening agents that make for a more chip-resistant casting, others have fillers and binders that lower their cost, while still others are formulated to maximize their absorbency for slip casting ceramic products or specialized to minimize their coefficient of expansion. |
| Buy plaster from ceramics suppliers in 100 pound bags. It costs between 7 and 18 cents per pound. Because it is hygroscopic, it will absord water from humid air and this will cause it to 'perish' (get too old and gritty to work with ) So, plaster should be stored in an airtight container- a 16 gallon rubbermaid container will hold one full bag. On every bag of plaster are big letters that say WARNING! MAY CAUSE SEVERE BURNS. This is because as plaster sets it generates heat. The exotherm, as its called, doesn't really set in until after the mixed plaster is pretty hard. Why I'm telling you this is the same reason USG prints the warning on the bag, many people think they can make an easy mold of a hand or a face or some other body part by sticking it in a bucket of plaster or slopping plaster all over it and waiting for it to set. Just after the plaster gets too hard for you to pull your self out it begins to exotherm. And it gets hot. |
| You will need buckets to mix your plaster in. My favorite are those high density polyethylene 2 and 5 gallon buckets you can buy at the hardware store. Be sure to cut the reinforced and ribbed rim off the buckets. This makes the bucket more flexible for breaking out set plaster dregs, less likely to crack. I have buckets like this I have used for 5 years. |
| USG offers tables that will show you precisely what weight of plaster to water will get you the best compressive strength. But most experienced plaster workers do it by eye. Mixing plaster is a pretty straightforward proposition, first get out one of those plastic buckets I told you to buy and fill that bucket to a little more than half full with water. This will give you a full bucket of mixed plaster once you've added the plaster. In fact, however much plaster you think you might need you will have to start with somewhat more than half that amount of water as the plaster you add will almost double the volume of the water. Using your hand, start adding the plaster to the water by sifting it thru your fingers handful by handful. You want the plaster to sprinkle onto the water's surface to ensure that it gets thoroughly saturated. This helps to minimize air bubbles in your mixture and gives you a better bodied plaster. (for casting masters, we use a flour sifter to ensure perfect saturation). AS YOU ADD THE PLASTER TO THE WATER, DO NOT STIR IT!!! When the level of plaster remains just a little higher than the water's surface, hold up and let the mix settle a bit, adding more plaster when there is clear water visible. Ever seen a mud flat? You know, when the water starts to dry up and the cracks start to appear, well, you want a small island of plaster that won't settle below the water to look like that and then you'll know you have enough plaster in your water. When you want to minimize air bubbles in your plaster, say for casting a master, you can let the plaster sit for five minutes BEFORE mixing it. |
| The rate at which plaster sets is directly related to how long it is mixed. The longer you mix your plaster, the faster it sets and the more homogeneously it sets. This kind of mix is very desirable when what you're doing is casting a pattern or a master, it makes for easy, controllable retouching and carving. However, when what you're doing is making a plaster casing or other part that you will be laying up by hand you can stir the plaster very briefly. By lightly mixing your plaster you will be given up to fifteen extra minutes of precious time in which to work the plaster. Basically, it is this central fact that you must work hardest to overcome. Once the plaster is mixed, it is going to set. And the plaster doesn't care if you're done or not. The oil in your skin, the dirt in the bucket, the minerals in the water, salt, chips of set plaster, even looking too hard at it, will accelerate the setting of mixed plaster, and cut into your work time, whereas lemon juice, vinegar, soft water and anything else on the acidic side will retard the setting of your plaster. A little too much acid and it won't set at all. Do not try to buy time by putting lemon in your plaster. It will only get you weaker plaster that will be sheer hell to work with. |
| Plaster mold casings (the 'mother mold' that holds the rubber) are made strong by reinforcing the plaster with 'Sisal'. Sisal is the raw hemp fiber from which manila rope is made. Look for it at construction materials supply houses. They may call it hemp, or casting fiber or some other term. A bale of Sisal will set you back about $150 but will make 100 or so average sized molds. |
| To make a mold casing with plaster, you will need to do it in layers. A putty (or 'face') coat of pure plaster, topped by 1 or 2 layers of reinforcing sisal, with maybe a 'gloss' coat over that to smooth it and make it pretty. You do NOT want to use separate mixes of plaster to make the separate layers of the casing. Because plaster expands when it sets, the second layer will continue to expand after the first layer has set and cause the mold section to curl and warp. Instead, you will have to use a single mix of plaster to do all the different layers and you can do this by 'fractioning' your plaster. |
| Here's how: after I sift the plaster into the water, I don't mix it. Instead, I pour off a section into a separate bucket and stir this up well, leaving the remainder in the original bucket unstirred. You can then apply this first section to the mold, flopping it over the surface by hand, creating a nice clear putty coat. The stirring action accelerates the setting. This will take some patience, you will have to stand and wait for the plaster to thicken a little so that you can build up the putty coat to at least a quarter inch thick. As soon as it is a little firm, go back to the original bucket and pour off another section into another clean bucket. The unstirred plaster is still liquid. Don't even bother to stir this second section, since the act of dunking sisal in it will stir it up enough. Use this section to apply a mat-like layer of plaster soaked sisal over the entire putty coat to reinforce the plaster. Because the first section has thickened up, you will get no sisal coming thru the putty coat. Once you have covered the putty coat, apply a thick, ropelike layer of plaster soaked sisal just around the edges of the mold section. If the mold will be cast upside-down, use the thickening plaster and sisal to form feet for it to stand upside down on. You will have to move pretty quick to get all this done before the plaster gets too thick to work. Keep in mind that plaster sets faster on your hands- keep a rinse bucket full of water nearby and rinse the plaster off your hands and tools between each layer. DO NOT RINSE PLASTER OFF IN A SINK. It will set in the pipe and that will mean digging it up. As the second section begins to get too thick to work, go back to the original bucket, HEY! still liquid! Use it to do any last minute sisal work, to finish the casing and level the feet. Square up the thicker edges of the mold- in a multipart casing, make them so square that a 'C' clamp can hold the mold together. |
| This technique can double or triple the working time of plaster. Because the entire body of the plaster is "one mix" all three sections achieve ultimate cure at very nearly the same time and all have almost identical coefficients of expansion. This minimizes or eliminates the warping that occurs whenever you laminate two separate mixes of plaster. |
| Other considerations in working plaster: Avoid acute angles- try to bisect every angle and separation line. A square edge is stronger and less likely to chip or crack than a narrow pointy one. Try to make the edges uniformly 1 1/2 to 2 inches in thickness, keep the field 1/2 to 3/4 inch thick. This helps reduce warpage and distortion and makes for a far stronger part. |
| To separate set plaster from a new section of plaster, like along the separation line of a mold, you must apply a separation agent. For plaster the ideal separation agent is Mold Soap. You can get Purelube mold soap from any ceramic supplier. Soap is ideal for ceramicists because it will not 'transfer' to the cast part- that is, plaster poured or cast against a soaped surface will not pick up or absorb any trace of the soap. This retains the plaster's ability to absorb water. Soap can be a pain though because after you lather up your separation line with soap you must wait for nearly half an hour for it to dry before proceeding. For this reason lots of people use VASELINE to separate plaster. Slather it on- then use a paper towel to get as much back off as you can, leaving a thin 'sheen' of grease. You can then immediately cast plaster against this surface. Do not wait an hour- the plaster will absorb the vaseline and you will no longer have a good separation. |
| I always recommend sealing the plaster first by painting the separation line with 'Orange Shellac' (the real stuff with beetle wing bits in it). This stops the plaster from absorbing anything, and it gives you a visible 'orange' line to help you gauge how thick the edge of your plaster is as you lay it up. However, be sure you don't forget to vaseline or soap the shellacked surface, because, if you don't- shellac will give you the most perfect BOND between two pieces of plaster. |
| Indeed- if you ever want to add plaster to an existing plaster casting, you have to seal the area you want to add to with shellac first. This will keep the old plaster from sucking the water out of the fresh plaster- allowing it to properly crystallize. If the casting is new and still fairly damp, you can patch or add plaster to it by soaking the area in water first, again, so that the old plaster (pre saturated with water) will not suck water from the fresh. |
| To patch bubbles and small flaws in plaster, open them with a fine pointed steel tool. With a small amount of mixed plaster, a small steel spatula, and a bowl of water and a sumi-e brush, wet the hole with water, spatula in some plaster, use the wet brush to brush off the excess and shape the plaster. |
| Also- be aware that many bubbles in plaster castings from rubber molds are caused by water beading on the surface of the mold. The surface tension of the water prevents the plaster from mixing with the water bead. Once the plaster has set- it sucks the water up, leaving what looks like an air bubble. Combat this by rinsing your mold in a surfactant, something that causes water to sheet instead of bead. Perma-Flex Mold Co. of Columbus Ohio make a product called mold dressing for just this problem. |
| If you are making a WASTE MOLD, that is, a mold made entirely of unreinforced plaster that you will chisel off of the casting, then add a mortar dye pigment to the putty coat as you stir it. When you are chiseling off the mold later, the color coat will warn you when you are getting close to the surface of the casting. |
| All in all, the best all round, multi-purpose plaster I have ever found is USG's Puritan Pottery Plaster. If you will be running a one-plaster bin shop, fill your bin with Puritan. |
| Christopher |
