Skin is a medium like no other, it can't easily be simulated and there are no rubbers, every movement you make results in an indelible mark that can only be removed through laser, surgery or covering it up with more ink. I've outlined a few reasons why I think skin is one of the most difficult mediums to work in for an artist.
The canvas moves
If you had a pencil and paper in front of you and started drawing the paper doesn't move unless you move it, or you're drawing outside on a windy day, if it did I'll bet you'll end up with something different to what you're attempting to do.
When you're tattooing you're tattooing a human, a body that although may try to sit still, is prone to moving as a result of pain, breathing, coughing, being distracted and in extreme cases reaching for a mobile phone (it's pretty rude but it happens alot, a client's phone goes off and they try and subtly reach into their pockets to get it thinking their slight body movements have no bearing on your job). Sometimes tattooing can be like trying to draw in the back of a 4*4 while going over rough terrain.
The canvas needs stretching
Although people think the needles hit the skin with such force it just naturally puts ink into the skin the reality is the skin needs to be stretched tight to offer resistance for the needles to penetrate, this is why tattooing needs both hands for the job, one to stretch the skin and one to actually tattoo with. You also use the stretching hand to gauge vibration as feeback from your machine to feel how it's hitting the skin. It's an all hands on deck art.
The canvas bleeds
Pretty much exclusive to tattooing, your canvas bleeds. While you're trying to inject ink under the skin you're making holes in it which allows blood to the surface, too much bleeding and you'll find it carries with it some of the ink you're trying to put in. It also means you're constantly wiping the skin while trying not to wipe away your stencil. Blood can carry pathogens so at the same time you're making sure you work cleanly and hygienically to stop and transfer of pathogens if they exist. It makes tattooing a hell of a lot more difficult than if you were to be drawing or painting the same design on paper or canvas. Like painting, it can get messy.
Skin ages and is prone to the weathering of the environment
She was once a pretty lass (we just can't see her face anymore...)
All mediums age, that's the funny thing about time, it affects everything in its' realm... but none more so than skin, Paintings and illustrations can be preserved for hundreds of years under the right conditions. Tattoos live in the real world, they're subject to sun, cuts and scratches, wear and tear from everyday work and the regeneration of the skin layers. As the body ages the tissue and volume of the skin changes and this affects the tattoo. Lines become a little fatter, colours can fade when exposed to UV rays. There's an added consideration when designing a tattoo to make sure it ages well, that lines are too close that in time they'll merge and that light colours aren't in abundance in areas of the body that get a lot of exposure to sun light.
You're usually not working on a flat surface
If you paint or illustrate it's usually on a flat surface. Now there's some exceptions like grafitti artists where the architecture or shape of the urban item being painted might differ but for the most part we tend to see drawings, paintings and other forms of fine art on flat surfaces.
The body is not a flat surface. Take the arm for example, it has more of a tubular shape that funnels down towards the wrist. Along the way is you find muscles changing the shape of the surface, an elbow protruding and what we like to call the 'ditch' an area on the opposite side of the elbow that allows for bending the arm where the skin changes slightly. It's far from flat and even though you may design the tattoo on a flat surface, you're usually not tattooing on one.
Now I'm not looking to say it's the most difficult medium but I think it deserves to be up with there with sculpting and the likes. Next time you're getting tattooed have some consideration for your tattooer, they're working in a challenging medium, namely your skin.
Peace Out