How does plaster set

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Finding the Best Sculpting Tools. Some Tips for using Apoxie Sculpt. Resin Jewellery Making Tips. Mold Making Ideas to do with the kids. Creative Concepts for Apoxie Sculpt. Essential Fiberglass supplies. You will need buckets to mix your plaster in. My favorites are those high density polyethylene HDPE 2 and 5 gallon buckets you can buy at the hardware store or scrounge from a food-service company or hotel laundry they get detergent in them.

Be sure to cut the reinforced and ribbed rim off the buckets. This makes the bucket more flexible for breaking out set plaster dregs, and less likely to crack. I have buckets like this I have used for 5 years. Some people rub a light coating of petroleum jelly on the inside of the bucket every so often to facilitate removal of the old plaster, but I never grease my buckets—I find that, by avoiding scratching up the inner surface of polyethylene buckets, the set plaster just falls out.

Restaurant supply houses sell a container that holds about 40 quarts. Rubber balls, cut in half, can be used for small batches. Set plaster pops out by turning the ball inside out. This will work on rubber too. Fitted into a heavy-duty electric drill, this will scrape the sides and bottom of your bucket without scratching as much as a propeller-style or Jiffy mixer, the commercially-available stainless steel cylindrical mixers.

Most professional shops casting production quantities of gypsum products that I have seen or consulted with use an industrial screw mixer that mixes the material without introducing significant air. The mixing head is a tube with a wormy kind of crankshaft down the center. As long as operation is continuous, everything is fine—the instant you stop mixing, you flush the head with water and then break it down and spend about 5 minutes cleaning the screw and cylinder.

At one company I set up a mold shop for, they had an overhead conveyor feeding rubber molds to the mixing station. They used a mixture of sand, hydrostone and cement and poured 3, figures a day. The flush water has to go thru a set of settling tanks and a silk filter before going down the drain.

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.

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 through your fingers handful by handful. This helps to minimize air bubbles in your mixture and gives you a better-bodied plaster. We casting masters use a flour sifter to ensure perfect saturation. As you add the plaster to the water, do not stir it!

Ever seen a mud flat when the water starts to dry up and cracks begin to appear? 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 use of a vibrator motor on the outside of the bucket can help bring trapped air to the top, or failing that, one can simply kick the bucket gently as one waits.

The rate at which plaster sets is directly related to how long it is mixed. The longer you mix your plaster, the faster and more homogeneously it sets. However, when making a plaster casing or other part that you will be laying up by hand, then stir the plaster very briefly.

By mixing your plaster lightly you will be given up to fifteen extra minutes of precious time in which to work. This is the central fact that you must work hardest to overcome: once the plaster is mixed, it is going to set.

The oil in your skin, the dirt in the bucket, the minerals or salt in the water, chips of set plaster—even looking too hard at it—will accelerate the setting of mixed plaster, and cut into your working time, whereas lemon juice, vinegar, and anything else acidic will retard the setting of your plaster. A solution of sodium citrate is often used sparingly as a retarding agent, as are certain proprietary powders. In general, do not try to buy time by putting lemon in your plaster.

It will yield weaker plaster that can be sheer hell to work with. Look for it at construction materials supply houses. They may call it hemp, or casting fiber or by some other term. Others like to use fiberglass-either loose chopped strands or loosely woven scrim cloth—or 7 oz. In general, the woven materials are best for larger or simpler molds, the non-woven ones conform better to the concavities of a small or complex one.

Larger molds may benefit from more rigid reinforcement as well as the use of harder plaster, like Ultracal 30 to allow thinner molds. This is more difficult to work with, as the setting curve is more abrupt.

You can use steel for reinforcement if it is galvanized. Uncoated steel will rust and crack the plaster as it expands, wood absorbs water and acts similarly. Copper tubing, though relatively expensive, is easy to conform to a desired shape. Large sculptures can be made directly in plaster by constructing an armature—wood and chicken-wire are commonly used—and applying plaster-soaked cloth or sisal.

These are not suitable for outdoor exposure, however, since the action of rain will quickly erode the plaster. They may, however, be refined and used as models for molding and casting in other materials. The most common sculptural use of plaster is for making models, small castings, ceramic molds, and casings for rubber molds.

To make a mold casing with plaster, you will need to do it in layers. 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 would continue to expand after the first layer has set and cause the mold section to curl and warp. Instead, you have to use a single mix of plaster to do all the different layers. Instead, pour off a section into a separate bucket and stir this up well, leaving the remainder in the original bucket unstirred, then apply this first section to the mold, flopping it over the surface by hand, creating a nice clear putty coat.

Properly done, the action of flicking the wet plaster onto the surface of your model will force the material into all the crevices and cavities, excluding pockets of air. 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 somewhat firm, go back to the original bucket and pour off another section into another clean bucket. Since the action of stirring accelerates the setting of plaster, the unstirred plaster will still be liquid.

Use this section to apply a mat-like layer of plaster-soaked sisal over the entire putty coat to reinforce it. Because the first section has thickened up, you will get no sisal coming through 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. Plaster of Paris is a versatile modeling material that you can use to create detailed models with the aid of flexible molds.

The plaster dries solid and will take on the slightest detail of your mold, from the smallest lines to subtle textures of the originally molded piece. Plaster of paris does not generally shrink or crack when dry, making it an excellent medium for casting molds. It is commonly used to precast and hold parts of ornamental plasterwork placed on ceilings and cornices.

If too much water is present, the mix will take an extra long time to reach the creamy stage and then, all of a sudden, it will set overly fast. The piece will still have good homogeneity, but the set plaster will be softer than it would have been had the desired ratio been used. Ceramic powder and plaster of Paris are different. The difference between the plaster of Paris and ceramic powder is what is in them. Plaster of Paris is made up of gypsum and dries to a hard matte finish.

Ceramic powder is made from zirconium oxide and dries to a hard finish. Safety Tips. Never mix plaster of paris with your bare hands. Plaster of paris powder is extremely light and fine.

Caution should be taken to avoid getting the powder into the eyes and nose. Can clay be made with Plaster of Paris? Yes it can.