Mold Making for the Budget Conscious Sculptor
By Sondra Schwetman
Associate Professor, Humboldt State University
Sculpture mold making is one of my specialties. I work with students of widely ranging skill levels.
Sometimes an easy mold made with items purchased from the hardware store is all that you need: no special art store, high prices, or special-order items.
Following is a process I use and teach to students. Typically, students don’t need a sculpture mold that will stay intact through 100 reproductions or “pulls.” Not to mention they don’t have the budget for expensive mold making urethanes and platinum cure silicones.
Of course, this method may be used by other sculptors as well. Really it is perfect for any budget-conscious sculptor.
Silicone Caulking Molds
Silicone Caulking Molds may do the job. A Silicone Caulking mold is appropriate for use with wax, plaster, and concrete and fast setting rigid urethane plastics. The molds do “off-gas” styrene that will inhibit slow setting urethane plastics. I use this mold making process to help students understand the parts of a mold, so they can move on to more advanced mold making.
Sculpture Mold Making Definitions
Molds are constructed from several basic parts.
Release is the material that prevents the positive from sticking in the mold.
Parting Line is the point in the mold where objects transition from the upper side to the undercut or undercuts – think about the halfway or equatorial point on a baseball. (Fig 1)
Figure 1: Find the Parting Line
Keys or Registration Marks are bumps or other shapes on the mold that fit together like a puzzle to line up the two halves of the mold. (Fig 2)
Figure 2: Making keys with washer
Pour Spout – this is the opening from the outside of the mold to the inside. It is used to pour the material into the finished mold.
Mother Mold or exterior shell is used to prevent larger flexible molds from flexing out of shape. Plaster works well for mother molds. (Fig 3)
Figure 3: apply plaster
Casting Cavity: If you are not familiar with mold-making, consider an ice cube tray or Jello mold. Pour water or Jell-O into the “casting cavity” through an “open pour spout”. An open pour spout would be the entire top of the ice cube tray or Jell-O mold. The water freezes and the Jell-O sets similarly to other products you could use for casting the “positive”.
Positive – the positive is the cast product taken from the mold. Positive space means the actual object.
Negative – Negative Space is around the object. It is also the Casting Cavity. It is the opposite of positive.
Materials for Your Sculpture Mold:
- Tubes of 100% silicone caulking. Silicone smells strongly of vinegar. An Organic Vapors Acid Gases Respirator may be worn or consider constructing your mold outside or in a well-ventilated room. The caulking MUST be 100% Silicone. It does not matter if the caulking is white or clear. Often I find that clear is the best choice because the object can be seen fairly well. Don’t try concrete patch, black or any other color; they will not set using this method.
- Caulking gun
- 2 1-gallon buckets: 1 bucket for water and plaster mixing, 1 empty bucket for holding your finished mold during casting
- Liquid dish soap: Palmolive seems to work best.
- Disposable latex or nitrile gloves: have available several pairs. Note: the silicone tends to stick to the nitrile gloves more readily than to the latex gloves
- Mold board: This can be any board.
- Potter’s clay and clay tools
- Plaster: I prefer #1 Pottery Plaster
- Murphy’s oil soap (interior/exterior release for plaster molds),
- 5-gallon water bucket for soaking your plaster mother mold. Water also acts as a release for the mother mold or for that matter, any mold made from plaster,
- Sharp Exacto knife or box cutter
- Safety glasses and
- Item to be molded: Choose something simple for your first mold. A clay model of a raindrop, for example. Small objects can be molded from a one-part mold, objects the size of a light bulb and larger should be molded in a two-part mold with a mother mold.
- Vaseline, if your object is made from either glass or glazed ceramic.
- Optional, but recommended, Smooth-On Epoxy-Kleen available at www.smooth-on.com
Words to the wise: you only forget your “release” one time! It is very frustrating! Also, do not use a precious object for your first mold making experience, the silicone and water may damage it!
++ See below for additional safety facts.
One-Part Sculpture Mold
- Put a small circle of dish soap on the moldboard. This will allow your mold to be put down without the mold sticking to the surface.
- If your object is glass or glazed ceramic, rub it lightly with Vaseline.
- Fill a one-gallon bucket 2/3 full of cool tap water
- Add about one ¼ cup liquid dishwashing soap and mix.
- Use a caulking gun to decant the silicone into the water.
- You have too much soap in the water if the silicone slides everywhere on the object. In this case, start over. There is no fixing this.
- If there is not enough soap in the mixture the silicone sticks to everything including your gloves; start over!
- Clean off your object using a paper towel and I suggest using Smooth-On Epoxy-Kleen available at www.smooth-on.com to get your mold very clean. Epoxy-Kleen is also designed to take unset mold components off of your hands. (Fig 4)
Figure 4: decant silicone into water
- Wear disposable latex gloves or nitrile gloves. Put extra soap on nitrile gloves, latex gloves rarely stick. Dip gloved hands in the soapy water to coat them with “soap release”. Hint: Have several pairs ready in the event that silicone sticks.
- Working quickly, pick up the strings of silicone caulking and pat them into a patty. Don’t squeeze, but let the excess water run off. Pat the silicone around your object. The silicone should be 3/4 inch to one inch in thickness evenly spread over the object, so that the object is embedded in the silicone. Set the mold down on the soap circle created in step one. If the object is heavy it may sink in the material, don’t worry about this, your mold will still work. (Fig 5)
Figure 5: Pat silicone into pancake
- Allow at least 8 hours for silicone to set. No mother mold is needed for small objects.
- Using a sharp box cutter or Exacto knife carefully cut mold on parting line of object or close to the parting line halfway around the mold. The cut does not need to be exactly on the parting line because you are making a flexible mold. This cutting method provides a “hinge” or keys that register the mold.
- Carefully remove the object and cut a pour spout with a sharp Exacto knife or box cutter. The pour spout should reach from the outside of the mold to the casting cavity. Make it slightly funnel shape cut as this will help in casting your material. Your mold is now ready to use. (Fig 6)
Figure 6: Cut object free
- Use small rubber bands or a ring of tape to hold your mold together for casting. I sometimes also use a bucket to hold my mold upright for pouring and setting and to contain the messiness.
Two-Part Sculpture Molds
Steps for a 2-part mold for Larger Objects
- Determine the parting line (median) of the object. Mark this median (on your original) with a sharpie if it is glass or glazed ceramic, or a needle tool for wet clay objects. Important note: all unfired clay for this process must be kept wet (leather hard is too dry, keep it covered with plastic).
- Place clay buttresses around your object. These should be about the height of the parting line on the object. (Fig 7)
Figure 7: build up clay for fences
- Roll out slabs of clay around ¾” thick, 3” wide and 8” long. Lay these slabs on the buttresses to form a circle (or square, etc.) around and touching the object. Carefully push clay in until it is snug against the object at the parting line. Use a ceramic tool, if necessary, to make a tight bond. (Fig 8)
Figure 8: clay fence
- One inch away from the object scribe a ditch key ¼” wide x ¼” deep. This will serve as the registration mark for the silicone mold. A ditch key is a registration mark that encompasses the object for casting. It is a little like a gasket.
- One inch away from the ditch key scribe 4 or 5 circular keys using a washer or coin. These keys will serve as registration marks for the mother mold. (Fig 9)
Figure 9: first side ready to go
- If the object being molded is glass or glazed ceramic, it must be coated with a thin film of Vaseline. This is a release to keep the silicone from adhering to glass and glass-like objects. Other materials require no release.
- Fill a one-gallon bucket ¾ full of water mix and about ¼ cup of liquid dish soap. Squeeze a tube of 100% silicone caulking into the water. You know you have too much soap if the silicone slides everywhere when you try to apply it to your object, not enough soap and the silicone sticks to everything.
- Wear disposable latex or nitrile gloves. Rub soap on the gloves to coat them with “soap release”. Pick up the strings of silicone caulking in the bottom of the bucket and form them into a patty. Don’t squeeze, but let the excess water run off.
- Pat the silicone caulking on to and around the object, covering clay to 1” beyond the ditch key. This step is important. Then the clay (and ultimately the plaster) should protrude beyond the silicone caulking mold about 1-½” to 2”.
- If done incorrectly your mold will probably leak.
- The patting action ensures no air is trapped on the surface of the object.
- The silicone caulking should be ¼” to ½” inch thick covering the object completely and evenly.
- For a larger two-part mold, you may need several tubes of caulking. Add an additional ¼ cup dish soap to the bucket of water. (Fig 10)
Figure 10: silicone from bucket
- Allow 20 minutes for silicone to set enough so that it will not be sticky.
- Mix-up plaster according to your preferred method* and create the first half of the mother mold. It is helpful to allow plaster to set to “beta stage” so that it is controllable (beta stage is when the plaster may be picked up and placed without running). The mother mold should cover silicone completely and extend an inch beyond onto the round keys. (Fig 11)
Figure 11: mix plaster by slake method
- Allow 8 hours to set and be sure to cover your clay with plastic to prevent drying.
- After 8 hours flip mold over and remove clay. Ideally the half mold stays stuck in the plaster. (Fig 12)
Figure 12: replace pour spout
- Wash away clay that is stuck with water on a cloth or paper towel.
- Using a paintbrush, brush Murphy’s Oil Soap on exposed plaster flange to serve as a plaster release. (Fig 13)
Figure 13: Murphy’s oil soap
- Repeat steps 3 through 6 from to create the other side of the mold.
- Allow 8 hours for silicone to set.
- Separate mother mold using a rubber mallet on the seam. (Fig 14) Tap the mold around the edges.
Figure 14: Break open with mallet
- Carefully pull silicone pieces apart. Remove your object. Your mold is now ready for use. (Fig 15)
Figure 15: finished mold with object
*MIXING PLASTER: ONCE SETTLED DO NOT TOUCH THE PLASTER, DO NOT TOUCH THE WATER, DO NOT TOUCH THE BUCKET! Surface tension is needed to keep your plaster from setting too fast.
I use the slake method which is a bit easier and requires no scales.
Fill a 1-gallon bucket ½ full of COLD water.
- Quickly sift plaster powder through your hands to find rocks in the plaster, discard rocks. I prefer #1 Pottery Plaster.
- In adding the plaster powder to the bucket we are filling the volume. The volume is full when a thin crust of dry, un-sinking plaster covers the entire surface of the water in the bucket at a fairly shallow depth.
- Allow the bucket to sit untouched for 2 – 5 minutes.
- Then carefully mix by sliding your hand down the side inside your bucket.
- Carefully break-up lumps; this should be all you need to do when mixing.
- Don’t mix it in such a way that you are whipping air into it.
- Now it is ready to pour.
- Don’t wait too long or your plaster will begin to set-up.
Additional Safety Notes
++ Important safety note: Silicone Caulking is not skin safe. Always wear gloves, safety glasses, and appropriate respirator or ventilation.
++ Plaster contains elemental silicon; wear a respirator or dust mask.
This guest post was written by Sondra Schwetman, Associate Professor, Humboldt State University
All images and content are copyright to Sondra Schwetman, used with her permission.
For more information about Sondra and her work, please visit her website: SondraSchwetmanArt
and be sure to find her on Facebook.