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!

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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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    34 Discussions


    Question 3 months ago on Step 10

    Hi there! I have a possibly dumb question. Does there need to be a parting line, or could you build a well deep enough that you could place the whole part (lower part of jaw the first time, and then upper part of the skull the second time) in and have your sprues come out the back? For instance, build a well deep enough, pour some of the mold media, gently place the bottom portion of the jaw on top of it, and while holding it in place, pour the rest of the mold media around it? Thank you in advance!


    4 years ago on Introduction

    Could you possibly post photos of the process of casting the top part of the skull?

    I have a cat skull I want to replicate but I'm not sure where to put the dividing line due to the eye sockets...

    1 reply

    Reply 4 years ago on Introduction

    Hi thanks for your comment. The photos are on an older hard drive. I'll see what I can find.

    To explain in words, how I did it, I put the parting line in the middle of the eye socket. So just like with the lower portion of the jaw, I buried the top portion in clay up until I had covered the middle of the eye socket (I filled the interior of the eye socked with clay up to that line too).

    I really hope I have pictures of this! If not I can probably get some together, or I will draw on the skull I have to illustrate it for you.

    Absolutely. Just ensure that whatever casting media you use is compatible with the mold media you're using, and the same goes for your mold release: make sure it is compatible with everything (good vendors will help you with this). The silicone mold media I used is compatible with resins.


    Thanks for reading! I definitely posted this all a while ago but haven't stopped working on it every now and again. Exploring some different avenues to get to a metal final product. Let me know if you have any questions!


    Very nice instructable, just what I was looking for! Great stuff...I have a few questions though...

    I couldn't get my hands on a coyote skull, but i just made a cat skull, fox skull and muntjac deer skull molds (horns/ antlers proving a big problem...) and i'm using mine to cast resin...

    Made mine same way---biggest problem I'm having at the moment is the teeth....they won't cast properly, you having same problem and did u find a way to solve it? The other big problem being the 'sprue' bits- tried to make small as possible (i used steel rods with clay) so as to minimise lost detail and having to cut/ sand them off- anybody found a way around that? others sell cast resin skulls and they're perfect....reckon they might have some special machinery though....;-)

    4 replies

    Hi Jake! Always glad to help. I have been experimenting with ways to increase fidelity on the teeth. I totally agree that they are a tough spot to get to come out right.

    For the teeth, what I have been doing recently that has worked better, is taking off the top of the mold, and pouring the wax directly onto the lower half. This way, the wax is still very much a liquid when it reaches the teeth. Otherwise, I was worried the wax was cooling as it was working its way down the sprue, and was not liquid enough to reach the bottom of the teeth.

    Another thing this helps with is removing the air that gets trapped beneath the wax. Since the wax is so heavy, if there is air beneath it, the air can not get out and wax can not flow in. Once the half-mold is poured, and still a liquid, I can agitate the area to try to coax the air out. Casting in vacuum (if you could rig it could REALLY help with this).

    I think both these solutions have the potential to improve the quality of your resin castings. Consider getting a brush, and moving it around in the mold-cavity where the teeth are to get the air out, and really make sure the resin is getting into the cavities.

    Even better than this, would be building in some channels that could carry air away from the teeth as wax flows into the mold (but you would need to redo the mold, and be very methodical about where you place the new sprues).

    Regarding the sprues, I could see a solution being to put your sprues somewhere else on the skull, in an area where cutting them off will not be super noticeable (such as on the underside, and not the outer cranium). Professional casting businesses probably do have a defined way they touch up the piece--it may be worth investigating how they do it!

    I hope what I've said makes sense, and is helpful to you. Feel free to post some pictures of your work, if that would help me to better answer your questions. :)


    Hi Brendan and many thanks!
    I have already tried what you suggest TBH. I can see how that works with wax as it's a lot less fluid as it starts to cool...unfortunately resin remains fluid for many hours before it sets, so just doesn't work. Actually came out a bit worse when i tried it, led to more bubbles and more flashing. On that subject...where did you get your mold straps from? other problem I'm having- I'm UK and bit limited to what i can get off ebay at the moment- tried gaffa tape but nothing sticks to silicon- and i am getting lot of flashing and not fill to top of the mold always because of it.
    If it wasn't for your work i would have tried to cast the skulls vertically/ sprue back of the skull...but you dnt want massive sprue off the cranium, so done it like you have....bloody awful things to try and get off though! Silicon is very pricey- can't afford to bin all that! Think i paid lot less for my silicon than you did...but i think the only option now is to try and drill air holes around the teeth. Whole point of trying to avoid that was that the teeth are the most detailed parts/ and where u place the mold line.
    I reckon those vacuum casters must be really pricey! + i can't find anything online- obviously wanna suppress that knowledge! ;-)
    i make my own mold release as well...melt one part vaseline and mix with two parts white spirit/ turps....and just brush on. very cheap and works well ;-)
    I'll try and add some images so you can see what i got so far...


    Hi Jake, two things regarding your comment:

    Mold Straps: I actually have been doing my castings without straps recently. When I was using straps, I used really large rubber bands (though in the UK, I'm not sure what you would call them! Perhaps they are also called elastics?). If there are any art stores near where you live, I would recommend going in and asking them what they have to hold molds together. Straps that work the best would be "ratcheting," "lashing" or "tension" straps. I did a search on the UK version of amazon for "mold straps" and got nothing--whereas the US Amazon has lots of results for "Mold Straps". But try "ratcheting" or "tension" straps in the UK amazon; I was able to find analogous products through there that way. It will probably give you nice results. These straps are better than rubber bands or elastics because you can set the tension in the line, and through that you can decide how tight your mold is held together. I found when using the elastics that the mold was too tight, and the thinner features were not coming out well.

    Vacuum Chamber: These are actually not so hard to make! And I think that this could be the missing link to your project coming out marvelously. Take a look at this instructable for a DIY approach--they look pretty inexpensive to make. Basically, you can take all the air out of your mold and do the casting in vacuum. No air in the mold means that your resin can completely fill the mold cavity. It seems like this is the problem both of us are facing. Air-hole/vents around the teeth could also be a work around to making a vacuum chamber. I am nearly certain, though, the companies that are making cast skulls are doing so using vacuums.

    Always wishing you the best, and always let me know if I can be of more service to you! Hope this all helps.


    ah...can see the link now, ta! didn't show up when it came though as a pm.....beyond my means at the moment/ no space....and other problems...but also looks bit too small for what i need now, but hope to come back to it later is things ever work themselves out! ;-)


    for the horns on the deer skull....bit of a risk! but i ended up having to cut slits into the mold to get the castings out. only did it that way in first place to avoid the mold probably shouldn't have done that!

    2 replies

    Looking good!! Thanks so much for posting pictures.Unfortunate about having to cut the silicone to get out the horns, but you've got to do what you've got to do. I imagine if you tighten the mold up when casting, the slit won't matter too much and you'll likely end up with a small parting line there. I wish you the best of luck continuing your project. Let me know if I can help you out any more.


    Thanks Brendan
    reminded me i needed to do abit of an update here anyway, so might as well be now ;-)
    Actually, the slit for the horns works very well...better than the alternative of having mold parting line there. When the top part of the mold is on it forces/ keeps the bit around the slit tight. RE: the teeth...drilling holes in silicon does not work....they just close up again, you can't physically remove any just goes through, if you see what i mean- so apart from hacking with a scalpel no chance- and u wd ruin the mold doing that! Only way I can make it work is to pour resin intot he teeth and let it set almost completely, before carefully assembling the mold and pouring as normal. It Kind of works...but very messy! To fill the teeth that way and get it back together without them falling out- net result lots more flashing, which can cope with better than non filled teeth. What also helps is pour out whatever left resin I had from the teeth into the horns...and tilt the mold up a bit so they fill properly....then you do get a complete and full cast- although with lots of annoying flashing.
    Mold straps- I had guessed they were basically velcro, so i just spent a small fortune on velcro strapping. ;-) Bit in two minds TBH- mixed results. Too easy to get too tight, or too looose, so soemtime shelps soemtimes doesn't. I think best on the lower jaw parts- where the top part of the mold is very thin, too thin really...about 10mm! If i ever remake it i wnt do that again....;-)
    Without straps definately works better on the main skulls
    We call rubber bands rubber bands int he UK to! ;-) Then again we just abt americanised now----we don't torture people either, just render the f*** out of them and....well, it's all for their own good really is it not?
    I couldn't see your link for the vacuum chamber though?


    6 years ago on Introduction

    Just wanted to say congratulations on being a finalists in the Concrete & Casting Contest! This was a fantastic instructable! Good luck and I still say...make it in white chocolate ;-)

    3 replies

    Reply 6 years ago on Introduction

    Also, to drop a little more research on you, I am not sure if the mold media I used is food safe (I presume it is, but I need to double check--I'll get back to you), however the manufacturer (Smooth-On) has a page on their site about using their mold medias for food applications.

    A food safe mold release (or no mold release) would need to be used. The mold release I used was particularly poisonous! Alginate (the stuff your dentist uses to mold your teeth) would be a really good temporary mold to use with food, but the mold would dry out and would need to be used with chocolate immediately.


    Reply 5 years ago on Introduction

    @poofrabbit: I did a similar instructable involving chocolate, and I can say from experience that white chocolate is very difficult to work with. I'm sure this depends somewhat on the brands, but ghirardelli (which is a pretty good brand) never really melts but instead becomes frosting-like. Probably not enough cocoa butter in proportion to sugar. Dark chocolate is another matter, and would easily catch the details as finely as wax.