Mulling paint is a time honored tradition that all artists and scribes alike must undergo. It is the process of suspending pigments in your chosen medium.
Wait... what do you mean you buy yours as a paste? From where? Wal-store or Hobby Entryway or Mike's?
Kids these days, not willing to put in the effort to make their own paint. Anyway, apparently people don't know how to mull paint, so I'll teach you. First you'll need a spatula/spoon/pallet knife, a plate, a muller, powdered pigment, something to put the paint in, and your medium of choice. I guess I should explain to the uninitiated what each of these is, but first a picture.
The pigment is just that, powdered pigment. They can be everything from a reddish dirt, to synthetic organic particles. I either buy pre-ground historic pigments or make my own. Here we are using real vermillion.
The plate is just that, a plate of glass or marble or granite or ceramic etc. The plate roughed up a bit with a grit of some kind. Silicon carbide, sand, diamonds, or whatever else you have lying around. (if you have diamonds just lying around please consider our custom work and we can work out a trade)
The muller is in the upper right of the picture and is kind of like a pestle with a large flat surface, which makes it better for dispersing and worse for grinding.
The palette knife is for moving the pigment paste around and scraping everything into the shell.
The shell is... well... it's a shell to be honest, not sure how else to put that. Sea shells are the traditional paint pots of the medieval world. If you notice a paint pot likes a bit like a better balanced shell.
Your medium is where you can add variety. If you are into oil painting, use linseed oil/walnut oil/poppy oil/not-corn-oil-because-it-won't-dry-ever, if you are into water color, you can buy water color medium, the same goes for latex, etc. I use a weak water color medium made from gum Arabic, honey, and water. It makes for a fine water color, but has the ability to be usable in egg tempera as well (my preferred medium).
Ok, enough about tools and supplies. Here is how it works. First you make a pile of pigment in the center of your plate. I know this is completely unexpected, but just hold onto your seats because it gets crazy from here. Next you put a little hole in the center of the pile like it's a tiny pile of toxic, powdered mashed potatoes.
The next step is to pour your chosen medium into the well you dug so daintily down...ok, enough alliteration. Pour the medium like the gravy into your tiny toxic pile. How much you use depends on the pigment, how fine it is ground, and how much you put on the plate. It's easy to add medium, it is hard to remove it.
Using your palette knife, mix the pigment into a paste, the idea here is to dampen all of the pigment so that it doesn't cause you to stir up dust with your muller. And because, well it's easier to mix with a pallet knife than a muller and if you do the easy bits easily the hard bits don't take as long.
Your next step, if you haven't given up already due to you preferring to buy your historic pigments from these people, is to begin mulling your pigment into the medium, by sweeping your muller on the pile of pigment paste in a figure eight and/or swirling motion. You have to use your pallet knife to scrape the muller periodically and to move all of the pigment back into the center of the plate. Depending upon the pigment and medium this takes between 5 minutes and 5 hours. (remember the people who do it all for you?)
You mull the pigment until it is a smooth paste, a little thinner than toothpaste, and ideally less gritty. This can take a while. As I mentioned before, you can add more medium to get the right consistency.
Once everything is smooth, consistently and flows the way you want, use the palette knife to scrape the plate and muller and transfer it to a shell or paint pot or tube or whatever you want to use to store it. I prefer a shell as I let it dry out and have a watercolor/half-tempera cake ready for me to use for illumination. Just add water and egg yolk/glair and I'm ready to paint.
Why would you want to do this you might ask? Well, it is hard to get historic pigments already made up into the medium you want, and it is hard to guarantee it was done right unless you a.) do it yourself or b.) know a guy. The other reasons are because you WANT the experience of making your own paints from start to finish, or making your own pigments, which sure you COULD send them to some friend of yours who writes blogs about this sort of thing to make paint for you, but wouldn't you rather experience it from start to finish?
Oh, wait, you don't make your own pigments? We'll have to work on that later.