Introduction: Cast a [Coyote] Skull

About: When I'm not studying engineering, I'm helping people reach the top shelves at grocery stores.

Hey there! In this instructable I am going to detail how I cast a coyote skull in wax. The methods for casting that I detail in this instructable can really be applyed to any skull, or geometry. I will try to outline why I made the choices I made so that you can make informed decisions for your own projects.

You can imagine that once you have this wax copy, multiple things can be done. For instance, one could do lost wax casting and make a metal coyote skull! Keep a lookout for my lost wax casting intractable which I hope to put out by the end of summer 2013.

Before we start, I will assume that you have a clean skull. I found mine out in the desert of Texas, and it was already very clean. There are also websites on the internet where you can purchase skulls, and local taxidermist may also carry some in stock.

Things you will need:
-- A clean skull
-- Material to build box (MDF Particle board, plywood, acrylic, anything thin yet sturdy)
-- Mold Media (I use a silicone mold media, will be covered later)
-- Sculpting Clay (be sure it is free of any chemicals that may inhibit the mold media from setting -- your mold media will let you know on the package)
--Mold Release specific to your mold media
-- Patience

So lets get started!

Step 1: Build a Molding Flask

In molding, the container in which you make and pour the mold is called a flask. The upper and bottom portions are termed the "cope" and "drag." 

The molding flask just needs to be a box with an open top. I make mine with a very thin MDF Particle board. I used this because I had some scrap on hand. It does smell kind of nasty and get pretty dusty when you use it and sand it, so always mind your respiration!

However, any thin, easily workable material will do. 

In terms of size, it needs to be big enough to hold whatever you're molding, and have enough clearance on the sides for the mold media. Whatever mold media you choose, it should come with recommendations on what type of space you want on each side,

Generally, the size of the molding flask will be the size of the final mold. This is because when we are done pouring the mold media and it has set, we will flip the flask over, take off the bottom board, and pour the other side of the mold.

Step 2: Stop and Think!

If you're doing a two part mold like I am, now is the time to stop and think about the mold design. 

Mainly, you need to put yourself in your shoes when you will be pouring the hot wax. Think about how you will want the wax to flow into the mold. Where are the points where air can get trapped and not allow wax to flow completely? Where will you want to include sprues and runners to allow the wax to flow into the mold cavity? 

Once you have this figured out, We will move onto casting the first half. I first cast the half that does not have any sprues and runners. In the case of the mandible (jaw), I cast the side with the teeth pointing up to be cast first.

Step 3: Prepare the First Half

To make the magic of two-part molds happen, we will embed the piece we want to cast half-way into clay. We will stick clay into the molding box up to wherever we want the parting line to be on the mold. 

CHOOSING CLAY: This is actually really important. If the container of your mold media says that you can't use any clay that contains a specific material then heed their warning! I used a sulfur free clay because that is what my mold media said to use.

On the jaw, I chose the parting line to not be completely horizontal because there were some deep undercuts and features I wanted to get out on the back near the jaw joint. 

I also stuck ball bearings into the clay so that when the clay is removed after one half is cast, there will be little nubs to help realign the mold during casting. See the picture to know what I'm talking about!

Step 4: Quick Break to Choose Mold Media

Before we cast anything, you need to choose what type of mold media you will want to use. 

If you have a lot of undercuts and delicate features to remove, you will need a softer mold media. In my opinion, you should go to a place that sells mold media and ask the people that work there.

I went to Reynolds Advanced Materials (there is an office near where I live) and they had tons of examples of castings for me to touch, as well as very knowledgeable and friendly staff. They also sell on the net.

In the end I went with a very soft Slicone Mold Media made by Smooth-On. It is very expensive, but very high quality. 

Step 5: Lets Cast the First Half!

So, we have the molding flask. The skull is inside, and it is embedded in the appropriate clay up to the parting line. You've inserted bearings to help with mold alignment, and you have the right type of mold media for your purposes.

Now is the time to use the mold release that is compatible with your mold media. WIthout mold release, the mold media may stick to whatever you are molding and never come off.

After the mold release is applied, mix the mold media however you are directed to (mine is 1:1 parts A and B by volume). And you pour. It is best to start pouring slowly into the lowest portion of the mold. This allows air bubbles to escape easier because the really viscous mold media will be rising, and pushing the air out, rather than falling on the air in any pockets and trapping it.

I recommend wearing some nitrile or latex gloves at this stage, because the mold media is really messy, really sticky, and really thick.It is vey uncomfortable if you spill some and get it all over the table and your hands like I did.

Once the mold is poured, you should wait the specified time, and then you are ready for the next step!

Step 6: Prepare to Mold the Second Half

Now you should have a really awesome looking one-half of a two-part mold. But we need to cast the other half! [The reason mine is so dirty is that there was a fuzzy side of my molding flask. I flipped the material over to its smooth side so that residue was not left over]

There are many ways to do this, but whatever you do you need to be careful. Whatever you do, try not to separate the bone from the already cast mold. This will ensure than the next round of mold media you pour in does not flow into those cracks.

Essentially, though, you need to remove the clay and the bearings from other side, and reassemble the molding container so that you can make a mold of the other side. 

You can use a brush to remove some of the clay if it sticking to the mold and bone.

Next we will attach sprues to allow the wax to flow into the mold cavity!

Step 7: Attach Sprues

The sprues are super important because they are the channels through which the wax will flow into the mold media.

Good sprue design will allow wax to flow into the mold so that it does not trap air, and they need to be big enough so that wax can flow in before it all cools (it cools and stops flowing quickly!)

I just take some of the leftover clay, roll it up, and stick it on. Its that easy! But placement and size do count. I really should have put the sprues also on the highest point of the bone. Since I didn't I lacked fidelity because some air got trapped up there. Its hard to explain, and is a sense that certainly comes from experience.ds

Step 8: Mold the Final Half

Again, just as before, apply a mold release, mix your mold media, and start pouring!

Step 9: Release the Piece!

Once the mold has cured, you can remove the bone! 

Break down the molding container, and carefully remove one half of the mold.

You can then slowly wiggle the bone out of the mold cavity!

Step 10: Cast It in Wax!

Here is the really cool and really applicable part of all that mold making. In this part of the process, we are making a wax copy of the bone (either cranium or jaw). 

Be really careful here when you are melting wax. You don't want it to catch on fire. And you don't want to burn yourself.

For wax, I am using a microcystaline wax I found on Amazon.

And this step is actually really simple. All there is to do is spray the mold with mold release, melt wax, and pour it into the mold. Depending on whatever geometry you are casting, you may need to vary the speed at which you pour and you may want to rock the mold while you are casting it to swish the hot wax around the inside. You may or may not want to keep your mold strapped together. 

I found that using some really tight rubber bands stopped the wax from flowing into the really tight features. You may want to use a molding strap (available at art stores and online) to hold the mold together. 

When I didn't use anything to keep the mold clamped, it actually worked pretty well. You'll have to do post processing regardless (to remove the sprues) so any extra steps to remove a slightly thicker parting line are kind of negligible. 

Step 11: Remove the Wax Copy and Post Process

So now you've got a wax copy. Take if out of the mold! Mine came out fairly easily, but you'll want to be gentle.

There will be a parting line along the wax copy where the mold itself parted. You can touch this up and smooth it out by heating up some sculpting tools and melting/shaping the wax to the desired smoothness. 

You'll also need to remove the sprues from the wax copy (You will probably want to design different sprues if you are casting in metal which I will cover in a different instructable).

Step 12: Bask in the Awesomeness!

Take a step back and admire the finished product!

You may need to refine your technique for casting in wax, but the mold is there. And depending on the mold media you chose, the mold could last for a very long time so you can practice indefinitely! It took me about 5 castings before I had a good technique down.

Thanks for reading, and feel free to leave questions in the comments section!

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