Basic Two-Part Silicone Mold Making

Now that you have finished your bust, you can move on to making a mold of it. Molds are not essential for every type of sculpture. For instance, if you created a sculpture out of water-based clay and fired it in a kiln, it would become hard and turn into ceramic (like your dinner plates). While you could make a mold off of that, it isn’t necessary because it’s already finished and will last forever (unless you break it). We have asked throughout the sculpture course that you use an oil-based clay because it is easier to work with over a long span of time due to the fact that it doesn’t dry out. But because the clay will never dry or harden, you’ll need to make a mold of it in order to cast it in another material that will harden (such as plaster). 

When I was first learning about mold making, I created a sculpture out of oil-based clay and left it in my studio in Southern California—in the summer. When it got to be about 105* outside, the sculpture quietly melted into a puddle of goo. Luckily I had already made a mold of it, but if I hadn’t I would have lost all those hours of work! We certainly don’t want you to go through that heartache. That’s why we created this lesson, so you’ll learn how to make a mold of your piece.

The mold, however, is not designed to last forever. The more you use it, the more it breaks down. Store your completed mold in an environment around 70* F to prevent the silicone from cracking. The mold will (hopefully) be good for about 100 uses. If you plan on making more than 40 casts of your bust, it’s a good idea as a backup to make a second mold from one of the first two or three casts you create that you can use to create more casts with later on down the line. The second mold will be made in the same manner as the first–nothing changes, it’s just there as a backup if you want to make more than 40 of your original sculpture. 

We’ll be guiding you the mold making process with a video tutorial (about 1 hr. 30 minutes total, in two videos) as we go along, so let’s get started.

A note for first-time students of this lesson:

Mold making can be a very time-intensive process and requires patience and about $250 to get started (some products will last through multiple molds). We suggest that you begin with reading through the text and watching the video to get an idea of what steps are involved in this process. Then, when you’re ready, you can take it step-by-step. A lot of mold making is waiting—waiting for the silicone to dry, waiting for the plaster to dry. You can break these steps up into several days of work if it’s better for your schedule. When you get to a point where you can stop for the day, we’ll let you know. If you want to wait out the drying process then continue your work on the same day, that’s fine!

Now back to it…

Here’s a list of the things we’ll be learning in this lesson:

Part One: Supplies And Getting Prepared
Supplies
Exercise One: Read Part One, Review Supply List

Part Two: Making The Mold
Exercise Two: Read Text
Exercise Three: Watch Videos In Their Entirety
Exercise Four: Make The Mold
Step One: Create A Clay Bed For Your Sculpture
Step Two: Lagging Up
Step Three: Refining The Seam
Step Four: Cleaning The Seam
Step Five: Sealing The Clay
Step Six: Creating The Keys
Step Seven: Building The Wall
Step Eight: Prepping For Silicone
Step Nine: Mixing And Applying Silicone
Step Ten: Second And Third Layers Of Silicone
Step Eleven: Creating The Shell
Step Twelve: Adding The Hemp Cloth
Step Thirteen: Removing The Wet Clay
Step Fourteen: Leveling, Adding Pour Spout, And Releasing
Step Fifteen: Silicone And Shell For The Back Side
Step Sixteen: Removing The Wall

Part One: Supplies And Getting Prepared

Because the clay we’ve been using during the sculpture course tutorials is an oil-based clay, it will never become hard like ceramic, and will always be malleable—thus subject to possible damage. You’ll need to make a mold of your piece if you want to be able to keep it. The really cool thing about having a mold is that you can use it over and over again as long as you take care of it. That means that instead of having just one sculpture, you can have 10 or 50 or more! Imagine all the fun things you can do with multiple castings of one piece. Exciting stuff! So let’s get going.

This lesson is broken down into two sections. In the first section, we’ll talk about the supplies you need to buy for this 2-part silicone mold making process. In the second section, we’ll go through the actual mold making process. This process will take up to 8 hours in total, including some inactive drying time.  At the end of the mold making process, your mold will be encased in plaster and will look like this:

This is how the mold will be used to cast copies, encased in plaster. It is then pulled apart in two halves to remove the copy.

If you want to skip ahead to the second section now because you’ve already read Part One, click here

Plan on spending 30 minutes to an hour on each of the exercises and steps in this lesson.

Exercise 1:  Read Part One below, review the supply list, and watch the first 7:25 minutes of the video.

Acquire or order the necessary supplies in the next few days.

Supplies

Download the supply list here. Most of the supplies can be purchased on Amazon (see our handy supply list here), but you will need to find water-based clay at a local art store or on DickBlick.com and hemp cloth at a hardware store. 

Kent will walk you through some of the required materials in the video from 0:00  – 7:25 minutes of the video. The materials are also found on our supply list.

You will need to have the following:

  • Your sculpture
  • Hot glue gun and glue sticks

  • Mixing sticks
  • Plastic mixing cups
  • Digital scale (weighs both grams and ounces)

  • Water based clay (sold in 25-lb blocks)

One 25-lb block of wet clay

  • 2 pieces of foam core board

  • Knife or wire (to cut clay)

  • Two-part silicone. Smooth-on makes a lot of great products. In the video, Kent suggests using a tin-based silicone rubber that needs to be measured with a digital scale. If you’d prefer to skip the scale, you can instead purchase OOMOO from Smooth-on that’s a one-to-one ratio, so you don’t need to measure anything. The OOMOO is less expensive, but may not last quite as long as the tin-based silicone. The silicone Kent is using in the video is pink, but silicone comes in a variety of colors. You may get white, pink, blue, or even clear—the choice is yours!

  • Mold release

  • Hemp cloth

  • Plaster or Hydrocal

  • 10-15 disposable brushes
  • Xacto blade or box cutter
  • Clear urethane spray paint

  • Rubber gloves
  • Dust mask

  • Bungee cords

 

Part Two: Making the Mold

This mold making course is found on two videos.

Exercise 2: Read through the entire text below, which summarizes the mold making steps found in the video.

Making a mold can be a bit confusing, so make sure you read carefully. Mold making is also very rewarding, so take your time with this lesson. You’ll be happy with the end result!

Exercise 3: Watch both videos in their entirety.

The first video covers Steps 1 to 8, the second steps 9 to 16. Each video is over 40 minutes. 

 

 

You may want to watch one video one day, the second the next day or two.

Exercise 4: When you are ready for mold making, make your studio comfortable, get your favorite music going, and (deep breath) get started!

You should plan on taking an entire afternoon to make the first half of the mold—including two 45 minute breaks when the silicone is drying, and shorter breaks when the plaster is drying. To complete the first half of the mold, you’ll need about 5 hours.

First timers, consider having a helper to assist you—perhaps even the 2nd or 3rd time you make a mold!

For each step, we recommend you watch the indicated portion of the video completely, then go back and follow the video as you do each step. There are also several places in the process where you can stop for the day, then resume the next day. We’ve indicated those in the text below.

Step One:  Create a Clay Bed for your Sculpture (7:30-12:40 on the video)

~15 minutes

Once you have gathered all your materials, lay down a piece of foam core board onto a flat work surface. Using a knife or piece of wire, cut off some chunks of water-based clay and start to make a bed for your sculpture. The goal here is to not have the back of your sculpture lay flat on the work surface because that could cause it to change shape or lose detail you’ve added. Lay down 2-3” blocks of clay in a rectangular shape with a hole in the center. This will support the outer edges of the bust and allow for the back of the head to rest in the hole in the center without touching the core board. 

Step Two: Lagging Up (12:40-19:10 on the video)

~30 minutes

Remove your sculpture from the armature by unscrewing the pipe at the base of the sculpture’s neck and place it, face up, on your clay bed. Using more water-based clay, start building up (or “lagging up”) clay around the bust until it reaches the halfway point or mark. So if this was your bust:

The dotted line would be your halfway mark or seam line.

Using strips of clay about 1.5-inches  thick, build up clay to the seam line. Get the clay all the way up against your sculpture—no gaps! Make sure you have at least 2 inches of clay around your bust on all sides. This will be where you create “keys” for the silicone mold later on.

Step Three: Refining the Seam (19:10-22:45 on the video)

~10 minutes

Now that you’ve lagged up your piece, it’s time to refine that seam line. In other words, you’ll be cleaning up the clay around your seam line. Using a tool with a flat edge and working all the way around the bust, make sure the water-based clay is touching all the way around your piece. Ideally, you want the water-based clay to be as level as possible, so remove any chunks that stick up otherwise they will become part of your mold. When finished, the clay should come out from the sculpture at a 90-degree angle and lay flat all the way around.

Step Four: Cleaning the Seam (22:45-24:40)

~10 minutes

Now that your water-based clay has been cleaned up a bit, take one of your disposable brushes and dip it into a little bit of water. Gently brush all the way around the sculpture. This will help clean up and level out your water-based clay even more.

Step Five: Sealing the Clay (24:40-26:15)

~15 minutes

Let the water-based clay dry a little bit, just so it’s not tacky when you touch it. This will take 5-10 minutes depending on your environment (faster if it’s hot and dry, slower if it’s damp and cold). When you touch the clay gently with one finger, it should feel cold and leave only a slight indentation. If the clay is sticky or holds onto your finger when you lift it away, let it dry a little longer. It should not be dry to the point of cracking—it that happens, you’ll have to redo steps 1-4. Once it’s a bit dry, take the bust resting on the core board outside to a well-ventilated area. Spray the clay and bust with a little bit of clear urethane spray paint. Spraying the piece will help seal the wet clay and create a smoother texture so you can begin to add your keys.

NOTE: In the video, Kent is spraying indoors. We strongly advise that you spray outdoors. It is better for your health and avoids creating any workshop odor.

Step Six:  Creating the Keys (26:15-29:50)

~5 minutes

Keys or key locks are designed so that when the two sides of the mold come together, they fit into each other to make sure the seam line stays put and doesn’t move around. You can use anything you have laying around to make the keys. Something with a cone shape is best so that the silicone halves really lock together.

This is what the keys will look like eventually.

Some ideas for what to use for making the keys:

  • The back end of a chopstick or a steak knife
  • The end of a Sharpie marker
  • The end of a toothbrush or paintbrush

Using whatever tool you have decided on for making the keys, press it into the water-based clay surrounding your sculpture. Keys should be ¼” – ½” deep. You don’t want them too deep or else they will be difficult to use; if they are too shallow, they won’t lock into place properly and your mold may slip.

Make keys about every ½” or so. You’re better off having more keys than you need than too few.

Step Seven: Building the Wall (29:50-38:30)

~15 minutes

Plug in your hot glue gun and let it warm up. Grab your second piece of foam core board and cut it lengthwise so that it sits about one or two inches over your water-based clay. Score (cut) the foam core board with a knife on one side about every ½” or so—this will help it to bend around the piece. You don’t want to cut through the core board so press lightly when scoring.

With the score marks facing the outside, start bending your foam core board around the water-based clay. Now that your hot glue gun is warmed up, put a little bit of glue around the base of your curved foam core board so that it sticks to the foam core board underneath your sculpture. Do this all the way around until you have a nice wall built up. This will prevent the silicone from dripping down the sides of your mold and will save you from wasting an expensive material. Make sure the foam core board is tight up against the water-based clay.

Step Eight: Prepping for Silicone (38:30-41:03)

~5 minutes

Get your scale, mixing cups, mixing sticks, and silicone ready so they are close by. Start by zeroing out the scale. Put your empty mixing cup on the scale and zero the scale out again so the weight of the cup doesn’t affect your silicone measurements.

You’ll need to move onto Video 2 to continue the process. Not a bad place to have some water too! 

Note after Step 9 and 10 there is a 45-minute break while the silicone is curing to have a chance to have a snack etc. There is also some time while the plaster dries.

The remaining steps can be found on Video 2.

 

Step Nine: Mixing and Applying Silicone Video 2 (0:00-12:35)

~15 minutes + 45 minutes drying time

Using your mixing stick, take a bit of the silicone (part one) and add it to your plastic mixing cup. Measure how much you have in your cup using your digital scale. In the video, Kent has 347 grams in his mixing cup, so he needs to add 34 grams of the catalyst or activator (part two) to this cup. It’s a 10:1 ratio, so will need to do your own math depending on how much of part one of the silicone you have in your cup. You can measure out part two of your silicone in a separate cup, or like Kent is doing in the video, just add it to the cup while it’s on the scale and keep an eye on the grams until you reach the correct weight. Be very careful if you do it this way—it’s easy to pour too much by accident!

With a clean mixing stick, mix the two parts together in one cup. Make sure to scrape down the sides and the bottom of the cup so that all the material is fully mixed together. This is a very important step! Mix it very, very well.

Once your silicone is completely mixed, begin pouring it on your sculpture. Start by pouring it on the highest points first. It will run down and drip, but don’t worry—that’s what it’s supposed to do. By allowing it to drip freely over the sculpture, you’re making sure that the silicone gets into all the little nooks and crannies of your bust. Pour all your silicone mixture onto the bust, then using a clean disposable brush, start brushing the silicone over the sculpture. Push gently and get silicone into all the details. Don’t push too hard because you can damage your sculpture and it will show later on when you cast from the mold.

For your first layer of silicone, don’t worry about how thick it is. What you really want to focus on is making sure every part of the piece is covered with a thin layer of silicone. After you’ve made sure you’ve brushed silicone onto every part, let it cure (dry) for about 45 minutes.

At this point, you can stop for the day if you like.

Step Ten: Second & Third Layers of Silicone (12:35-15:10)

~15 minutes + 45 minutes drying time

~15 minutes + 2 hours drying time

Now it’s time for your second layer of silicone. Make up a new batch of silicone in the same way you did in Step Nine. Pour the mixture over your first layer and for about the first 10 minutes or so, keep brushing the silicone mixture, pushing the material from the lowest points (where it wants to settle) onto the higher points of your sculpture. If you don’t do this, the silicone will all settle into the lowest points (your keys and water-based clay bed) and you will end up with a very thin and flimsy layer on the important part (your bust). The silicone should be the same thickness on the face as is it around the keys. After you’ve done that, let it sit for another 45 minutes or so.

In the video, Kent needed three layers of silicone in total to achieve about a ¼” thickness on all parts of the piece. You may need three or even four coats. It’s always better to use more than you need than less than you need. A silicone layer that is too thin will pull apart and break, making your mold unusable.  Reminder: A 45-minute break is needed for curing between each coat.

After the third coat of silicone, you can stop for the day if you like. The silicone will need at least 2 hours to dry completely before moving on to the next step. Be sure to cover your sculpture with plastic so the wet clay supporting the bust doesn’t dry out.

Step Eleven: Creating the Shell (15:10-23:05)

~15 minutes + 30 minutes drying time

It’s time now to create the hard shell case for your silicone mold. Make sure you have a dust mask handy because it’s not good for you to breathe plaster dust. Some people experience sensitivity or allergic reactions to plaster, so you may also want to wear gloves when you mix it.

We will be using Hydrocal (a strong gypsum cement) and hemp cloth for our shell.

Hydrocal requires 45 parts water by weight to 100 parts plaster. There should be mixing instructions on whatever type of plaster you have, so check those out. Essentially, we want the texture to be like “fluffy toothpaste” as Kent says in the video. You may also think of it like soft serve ice cream.

First, put some water in a large, clean plastic mixing bucket. Start with about ½” of water in your bucket—you can always add more later if you need it. Scoop some of your plaster into the bucket a little at a time. In the beginning, the plaster will settle to the bottom of the bucket. When it starts forming a little pyramid or mountain on the surface of the water, start mixing it all together. Using your hands, squish the plaster and water together, making sure to smash all the little balls of clumped plaster between your fingers so that you have a nice, smooth consistency. When you hold up a bit of the mixture, it shouldn’t be runny or drip much, but retain some form. If it’s too runny, add more plaster and mix again. If the mixture seems too dry or lumpy, add a more water just a little bit at a time and continue mixing until it has the correct consistency.

Once your plaster has a good consistency, start adding it on top of your silicone. Start at the lowest parts first then work your way up. Wiggle your fingers through the plaster to get out any air bubbles. The plaster will start to harden up in about 10 minutes, so don’t dilly dally! You can smooth out the plaster slightly, but leave some texture to it so that the next layer has some points to adhere to.

Now is a good time to rinse out your mixing bucket. It’s easiest to clean out the bucket when the plaster is still wet. Plaster will clog the sink, so it’s best to rinse it out with a hose outside if you have that option available. If not, you can wait until the plaster has dried and throw the chunks into a garbage can. Dried plaster can be easily removed from the bucket by gently squeezing the edges of the bucket or knocking it against the edge of your workbench to release the dried bits of plaster.

When the plaster sets up, it will get warm and feel a bit moist. After this, it will cool and then completely set in approximately 30 minutes. Then you’re ready for the next layer.

Step Twelve: Adding the Hemp (23:05-28:45)

~15 minutes + 30 minutes drying time

Begin by cutting your hemp cloth into 6” x 6” pieces and wet them slightly. Wring out any excess water—you want them damp, not wet. The purpose of the cloth is to make the shell more durable. If you drop the shell and it cracks, the hemp cloth will help hold the shell together so it is still usable and can be easily repaired. Without the cloth, a broken shell would just crack apart into unusable pieces.

Make a new batch of plaster in your clean bucket (you don’t want hard chunks of dried plaster mixing in with your new plaster). When your plaster is fully mixed and of a good consistency (see discussion above), put down a thin layer onto your dried plaster. Dip the hemp cloth into your plaster bucket making sure to fully saturate each piece. Lay the hemp pieces down on the mold and rub them gently so that you break the air bubbles and the cloth adheres to the plaster underneath. Using any leftover plaster, cover the hemp cloth and smooth the plaster as it begins to cure so that you have a nice looking shell when it’s all completed.

FIRST HALF COMPLETED!

You can either stop here for the day or continue on making the second half of your mold.

For the second half of the mold making process, you’ll need about 1.5 hours of active time and about 2.5 hours of inactive time while your silicone and plaster layers dry. Let’s get started!

Step Thirteen: Removing the Wet Clay (28:45-31:40)

~15 minutes

After your plaster has had a chance to dry completely after about 30 minutes or so, we’ll be flipping your entire piece over to remove the wet clay that you started with.

First, using your knife, carefully run the blade along the underside of the mold to release the hot glue strips that were holding your foam core pieces together. Gently flip your mold over revealing the second side of your sculpture.

Leaving the foam core wall in place, start pulling out the wet clay until it has all been removed from the second side of your mold. You may need to use a damp brush to remove all the bits of wet clay from your sculpture and silicone. Save some as we’ll use it in Step 14.

At this point, you can stop for the day if you like. Because there is no more wet clay involved, you can leave the sculpture uncovered at this point. 

Step Fourteen: Leveling, Adding Pour Spot, and Releasing (31:40-35:40)

~20 minutes

Once the second half of your sculpture (the back of the head here) has been cleaned and dried, you can take it outside in a well-ventilated area to spray it with mold release. THIS IS A VERY IMPORTANT STEP. Do not forget the mold release! If you skip this step, the silicone poured on each side will adhere together and you won’t be able to open your mold. When you spray the mold release, you don’t have to spray a huge amount of the release (it shouldn’t drip) but do make sure you get a fine layer over every part of the sculpture and silicone. Any part that is not covered in mold release will adhere to the next layer of silicone.

At this point, you also need to make a sprue, which is an opening or pour-spout. This is where you will pour material in to your completed mold to make a cast of your sculpture. THIS IS ALSO A VERY IMPORTANT STEP. If you forget this part, you won’t be able to use your mold!

Reusing some of the wet clay that you removed from the sculpture in step thirteen, build up some clay from the bottom of your sculpture all the way out to the foam core wall around your piece.

Smooth out the clay so that it joins seamlessly with the bottom of your sculpture. When you’re finished, it should be a cone shape. Make sure the end that attaches to the foam core wall is at least 2-3” wide, so you can easily pour material (such as plaster or resin) into it later to create multiples of your bust.

Stabilize your mold by adding the leftover wet clay underneath it.  You want it to be level and solid so that it doesn’t tip over when you pour the silicone and plaster on this side.

Step Fifteen: Silicone and Shell for the Back Side (35:40-37:57)

~10 minutes + 45 minutes drying time for each layer of silicone, then 2 hours for final layer of silicone to dry before adding the plaster.

~15 minutes + 30 minutes drying time for each layer of plaster

Mix a new batch of silicone in a clean plastic container and pour it on this side of the sculpture (repeat step 9). Pour 2-3 layers of silicone just like you did before, taking 45 minutes between to cure the silicone. Once you have the first layer of silicone down, you can take your sweet time with the rest of the process. There’s no more wet clay to worry about, and it won’t hurt the silicone to have more drying time.

When the final layer of silicone is completely dry (after 2 hours or more), repeat steps 11 & 12 so you have a plaster shell over the silicone.

Step Sixteen: Removing the Wall (37:57-44:10)

~15 minutes

Now comes the exciting part! Once the plaster on your second side has dried completely, you can remove the foam core wall around the outside of your mold. After you’ve removed all of the foam core, remove both of the plaster shells and set them off to the side. Now gently pull apart the silicone and remove your sculpture and the wet clay. Clean your silicone by rinsing it off with some water. You may need to use a damp brush to get all the material out of the mold.

When you put the two sides of the silicone back together, you will see how the key locks work to hold the two pieces firmly together. Put the shell back on, and your mold is ready to go!

You can read about how to use the mold to cast your bust sculpture in our upcoming Casting Lesson (look for it in 2018!).

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