USG #1 Pottery Plaster
Warm water
Murphy’s Oil Soap
Small sponge
Formica counter top scraps cut into several shapes (to be discussed)
Small Spoon with a somewhat pointed end
An old dinner knife
Wooden potters tools(kemper WT19,WT22, WT26, or similar)
White ceramic clay
White, natural bristle paint brush (cheap one) about 2” wide (from Home Depot, Lowes or other painting supplier)
A few feet of small Rope, such as laundry line – do not recommend nylon ropes or ropes that will stretch.
4 Small blocks of wood or formica scraps 1”x4”x3”
A small item flat on one side, such as a medallion or ornament or magnet.
Plastic bucket large enough to mix and hold adequate amount of plaster for the project
At some time during every ceramist’s career, they will want to make a mold of something. Sometimes it is just the challenge of learning how, other times it is to be able to reproduce something they created multiple times. Mold making is in itself an art form that takes some skill and practice to master.
The best place to begin is to learn the basics with a one piece, or open cast/press mold. This type of mold creates a product that is detailed on one side and flat or plain on the other side. Most often these molds are cast solid, but can be cast as a ceramic shell. Open, one piece molds can also be used as press molds for use with various clays.
Like the negative of a photo, a mold reflects the opposite of the product, so everything is in reverse until you press or cast it then the resulting item will be the positive match for the original piece.
Disclaimer - The approach I am about to lay out is but one way to make a mold for ceramic and clay use. The general principles may be the same, but some of the materials and steps may vary from mold maker to mold maker.
My approach is to teach the novice the principles of ceramic mold making in its simplest form
Supplies:
USG #1 Pottery Plaster
Warm water
Murphy’s Oil Soap
Small sponge
Formica counter top scraps cut into several shapes (to be discussed)
Small Spoon with a somewhat pointed end
An old dinner knife
Wooden potters tools(kemper WT19,WT22, WT26, or similar)
White ceramic clay
White, natural bristle paint brush (cheap one) about 2” wide (from Home Depot, Lowes or other painting supplier)
A few feet of small Rope, such as laundry line – do not recommend nylon ropes or ropes that will stretch.
4 Small blocks of wood or formica scraps 1”x4”x3”
A small item flat on one side, such as a medallion or ornament or magnet *.
Plastic bucket large enough to mix and hold adequate amount of plaster
Getting ready:
1. Select an item that is flat on one side, a medallion, a magnet, an ornament, or something similar *.
2. Pour about ½ cup of Murphy’s Oil into a pint jar. Add 1 cup of water. Close and shake well. Let it sit undisturbed over night. Without shaking the jar, carefully cast off about 1 Cup from the surface into another container Use bottom portion in making your mold. DO NOT shake before using as it will cause bubbles to form in your plaster casting.
3. Cut the scraps of formica counter top into varying lengths and shapes, keeping the following in mind: You will be building a modular, reusable box.
A. The bottom of the box needs to be big enough to have space around the item you will be casting. Very small items need an inch on all sides, larger items will need two inches of space on all sides PLUS you will need to add at least one more inch to each side to account for the sides of the box to sit on top of the bottom. Therefore, an item that measures 2” x 3” will need a box with a base no smaller than 6” x 7 “. Because this box is adjustable, the base can be much larger, but not smaller. I recommend you begin with a square of formica about 10”x10” (if your item is small enough) so you can use it again later with items that are a little larger.
B. Now you will need to create sides for the box. Again, depending upon the size of your item, you need a thickness of at least one to two inches more than the thickness of your item. Small items less thick, larger items 2 and sometimes 3 inches thick. If you are doing a medallion that is ½” or less thick, you can get away with a total of 1” thickness for the entire mold. So, keeping this in mind, you want sides that can be used again for other molds, but not so deep that you can’t work with the insides, so I recommend your sides be 10-12” long and 4 inches wide, based upon your making a small mold.
C. Cut four pieces of the formica or other wood scraps about 4”x3”x1” to use as wedges.
Now we begin
Step one: Assemble formica pieces to form your box. (See diagram on how to position sides.) The smooth, shiny side of the formica will be the inside surface of your box. The side pieces will sit on their 1” side on top of the bottom base and will come off each other at a 90 degree angle somewhere along the adjoining side. At each corner the two sides will form the shape of a T. You want to make sure the box will be at least 2-4” bigger than the item you are making.(Small items, about 1” space on all sides of the item, larger items 2-3” space on all sides.) See diagram.
Step two: Wrap a length of rope somewhat loosely around the four sides to hold them upright and tie firmly. Use a square knot so you can easily loosen it after the mold is made.
Step three: Using the small blocks of wood, create wedges between the rope and the sides (where they meet at the corner of the interior) to tighten the rope and hold the sides firmly in place. To do this, insert a block at an angle between the rope and one side of the mold, turn it to 90 degrees of the side, then slide it into position. Repeat on all sides. The last side/wedge will be very tight. (See diagram).
Step four: Place your item on the bottom in the center of the box. Verify you have adequate space on all sides.
Step five: There are two ways to anchor your item in the center of the box. For small flat backed items, I like to take a pat of clay and roll it out to a thin pat (about 1/8 inch thick) slightly larger than the item. Place it in the center of the box and press your item into it, details on top. Using your knife or one of the wood pottery tools, remove excess clay, being careful to not cut under the item. You want the sides to be perpendicular or slope out from and not under the item. A second method is to hold your item in place while you press clay into the seam/space between the item and the bottom of the box. You will need to scrape away excess and leave a clean smooth line, being careful to not cut away any of the clay from under the edges. I have found it is much better to let this clay slope outwards very slightly to prevent the plaster from hanging up on any ridges along the edge.
Step Six: Using your finger or one of the pottery tools, press clay in all corners and edges of the box, thereby sealing it. Smooth out the clay with your finger or the tip of the spoon. Slightly rounded edges are good.
Step seven: Taking the natural bristle brush, dip it in the soap solution and paint your item thoroughly, as well as the bottom and the sides of the box. Be sure to scrub it into all the details. Apply a second coat. Make sure it is smoothed out and there are no bubbles present. Blow any bubbles out or pierce them with something sharp.
Step eight: Compute how much plaster you need for the mold by determining the cubic measurement (volume) inside the box and how deep you want the mold to be (Height X Width X Length). The ratio of plaster to water is 70 parts water to 100 parts plaster, or 7:10. This equates to 1 part water to 1.34 parts plaster. Using a hypothetical size for your mold of 4” wide X 6” long X 2” high, you would compute your proportions like this:
4”X6”x2” = 48 cubic inches = .83 quarts = 26.56 ounces US = 3.32 cups volume = 1.66 pounds water or .753 kg (a quart of water weighs 2 pounds US)
1.66 pounds water x 1.34 = 2.25 pounds plaster, or
.753 kg water x 1.34 = 1.02 kg plaster
A more accurate and precise measuring can be obtained by using metric measures.
Step nine: Now it is time to mix your plaster. Once you add water, there is no turning back. How you mix your plaster is your most important step and will determine the strength and durability of your mold. Measure your plaster and set aside.
Temperature of your water is also important. Measure the determined amount of warm water (about 90 degrees) into the mixing bucket. Slowly sift/shake the measured plaster into the water by hands full and allow it to sink. Soak for about 3 or 4 minutes so that it can fully absorb water, and then begin mixing with your hand (for small batches – 5 to 10 pounds), until it forms a nice silky glove on your hand. It will feel somewhat like warm pudding. This will take about 10-12 minutes. Care should be given that you DO NOT OVERMIX. As it mixes, it will get warmer to touch. You don’t want the plaster too thin or it will not mature appropriately and it can splash and cause bubbles as you pour it. If it is too thick, it can cause air pockets and it will not fill in the details.
ALWAYS add plaster to water, NEVER water to plaster.
For larger production, it is important that mixing be consistent and it is advised to use motorized mixing. Size of motor would depend upon your production level.
Step ten: Once you have the plaster ready, pour a steady stream into the box, aiming for the corner or side and letting it fill and flow freely over the item. Fill the box to the appropriate level needed.
Step eleven: Immediately clean the bucket before the plaster hardens.. Let the mold set until completely firm.
Step twelve: When the mold has hardened enough to hold its shape and be handled, remove the wedges to loosen the rope and remove it. Knock the sides away from the mold and remove the mold from the bottom. Clean away all the clay. You can re-use this if you save it in an air tight container. Using the knife, scrape all exterior edges of your new mold, removing the sharpness.
There you have it. Let the mold dry thoroughly before you try to use it.
NOTE: DO NOT purchase large quantities of plaster unless you plan on using it over a short space of time. Find a source who will sell it in smaller quantities equal to your projected usage. Once it is open and has access to air, it will degrade over a very short period of time.
*DISCLAIMER: When making molds of items, be aware of copyright and trademark infringement. There are laws and penalties for copying others creations.
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