Bone vs. Resin vs 3D printing: Properties and workability
A common question I get is “why don’t you just buy/make fake skulls?”
I absolutely understand where the question comes from: an animal only has one skull, and using it means the death of the animal. However, I do have moral and material reasons for choosing the medium I do. This blog will talk about some of the working qualities of alternatives vs. natural bone.
Microplastics
Prefacing this article with a brief explanation, microplastics are a very real contaminant that damages ecosystems and poisons everything that consumes enough of it. Microplastics come from clothing, equipment, trash, and any plastic that breaks or wears down over time. Birds and fish eat this material all the time, and as they are predated upon the plastic accumulates in their predators and spreads through the environment.
This includes faux fur, which breaks into tiny pieces over time and is not biodegradable.
Resin casting and 3D printing also produces plastic waste, ones that cannot be recycled or reconstituted easily. Though these make it easier and more accessible to produce replicas, there are particular disadvantages to the material that influence my decision to stick with natural bone.
Structural differences
I will be discussing four different materials in this entry: Bone, Thermoplastics (PLA, PET, ABS), Cold-cast resins, and Photopolymer (3D printing) resin.
A carved wolf skull
The first material is bone. My preference for the natural material is mostly due to two factors: the anatomy and the biodegradability.
People who have known me for a long time know about my lifelong fascination with skeletons and anatomy. Part of my process in working with bones is interpreting the animal’s life and working around its natural structures. Sinuses (spaces) and thin bone are just part of the process, and an intimate knowledge of what areas have thick or thin bone adds a level of understanding to the process that not many people have. I embrace the differences between individuals, I never make the same carving twice partially because there’s no guarantee that I can replicate it!
Spaces in the skull can add visual depth; my usual openwork carving reveals these cavities in the skull that adds a sort of backdrop to the patterns. I cannot replicate these spaces by hand because they’re simply too deep!
Cold-cast resin is poured into a mold, allowed to cure, and then popped out and cleaned up. Though the material is strong, the structure of the skull is different once cast. The replica will either be solid resin or a hollow shell. To most collectors, the differences would mostly stop there. Carving a resin skull is fun in itself; resin casts can be made into functional objects without worrying about the porousness or integrity of the material. You also don’t have hidden gaps or sinuses to worry about.
3D prints use the same filetype as CT scans, so sometimes a given 3D model will have some of these structures and sinuses. If you’re out to collect specimens and don’t mind replicas, 3D printing is a great way to get access to some varieties that are impossible or illegal to own in their natural state. This may mean that there are pockets of un-cured resin within the skull, though, so be careful when carving these beccause some internal parts may be a little sticky.
Key Material Differences
Bone is biodegradable and bone meal makes a good fertilizer or soil additive. The bone dust produced from my method of carving is extremely fine and soft, making it useful as a resin additive as well. Spiritually, some people use particular bone dust in their totemism or pagan rituals. Bone dust is dangerous to inhale as the bone bind to organic material in the lungs and can cause respiratory issues.
Thermoplastics such as PLA, PET, and ABS are formed by heating the material, shaping it, and allowing it to cool and harden. Manipulating thi material requires heat and can give off toxic fumes or atomized particles.
Cold-cast resin (2-part mix) is a durable material that starts out extremely viscous, so it can pick up deep details when cast in a proper mold. When carved it does one of two things: it either produces a very fine resin powder, or the resin will come off in small shavings. The resin powder is not safe to inhale and will not biodegrade. If you accidentally mix some with soil it can leach something into the soil that makes some plants very unhappy.
Photopolymer resin (3D printing) is relatively new on the market. For collectors, the detail one can get out of photopolymer prints is incredible, and will usually suffice for personal collections or anatomical reference. The material tends to be fairly brittle and prone to shattering. Tenacious and ABS-like resins are available, but these only go so far to mitigate the problem.
3D prints are also limited in size depending on the machine used to print it. Large skulls such as bulls or moose cannot be easily replicated to scale.
Workability
As a carver of organic material, I’m always casting around for easy replacements for materials. 3D printing is something of a boon to my studio, as I can work with skulls. Over almost a decade of carving and experimenting, my preference is as follows:
1: Natural Bone
2: Cold cast resin
3: Photopolymer resin
4: Thermoplastic
Ultimately my preference comes down to two things: the type of dust produced, and the longevity of the piece.
Natural bone creates a relatively small amount of bone dust when carved, and the natural growth of the bone means there isn’t a shear point or a particular grain.
Carving cold cast resin tends to create expanding powder that responds to static electricity and sticks to everything!
Photopolymer resin, even ABS-like and tenacious, tend to be more brittle and prone to cracking under heat and pressure. Carving produces a very fine dust that is irritating to the skin and eyes.
Thermoplastic is my least favorite: friction is a part of the carving process, ince the burrs are essentially spinning metal blades against the surface. Thermoplastics tend to melt and deform with heat and pressure, meaning details may soften or parts may collapse if they’re worked too quickly. The resulting castoffs are stringy, sticky, and tend to fly through the air and stick to things.
Painting and Finishing
Generally, a good primer makes paint stick to anything and everything.
Bone tends to take paint best, provided the surface is free of grease and isn’t polished. Some dense bones like cattle or areas of the nose and face of a skull may be denser and less willing to take paint: you can lightly sand these areas with 400-600 grit sandpaper and you should be able to paint on the surface without any major texture differences. The most annoying part of working with natural bone is the natural grease. If you see yellow spots on a skull, it usually means there’s still some fats or oils left over that are leaching to the surface. You can treat this by soaking the bones in warm-hot soapy water (preferably with dawn degreaser) for a few days or leaching the grease out with acetone.
Grease leaching out from inside bones will eventually dislodge paint, sealant, and anything sitting on the surface of the bone. For longevity’s sake, make sure you remove as much grease and dirt from the bone as you can before you paint and seal the surface.
Cold Cast resin binds to alcohol inks and acrylics very well with or without primer. Resin is the easiest to paint on and color because of its consistency, and the material takes polymer dyes readily for bright and unusual hues. For the strongest and most consistent colors, this may be your best bet.
3D printed resin doesn’t absorb alcohol ink as well, but also takes paint and stains fairly well. I do not recommend using polymer dyes on 3D printed resin; heat and moisture will make the photopolymer resin crack and warp.
Thermoplastics tend to bind pretty well with acrylic paints, provided the paint is sealed afterward.
All plastics and resins can stain over time when exposed to pigments. Remove paints quickly if you don’t want a color there, or you might have to sand the surface to make it go away.
There are ways to work with any medium you choose; figuring out how to work around limitations is an important part of being a material artist. I have an invested preference for natural bone because of the way I work and what I prefer to deal with post-processing. Your mileage may vary!