I have in total about four different types I use and each has it's own unique properties and uses so thought it would be a good idea to make a series of posts about each one.
The first we will cover is "Brown Stuff". The company that produces it is called Polymeric and it is readily available on the Internet. Here is a quick link to a supplier : http://www.warehouse23.com/item.html?id=PSI011-24. Other suppliers include Rafm miniatures and Reaper Miniatures just to name a few.
Brown stuff is a two part epoxy putty, the white strip is the hardener and the brown strip is the softener and when the two are mixed the putty will cure in about 20 to 30 minutes. Normally you mix the two 50% white, 50% brown. As with all expoxy putties a trick to remember is that your ratio of hardener to softener will determine the eventual density when it dries. If you want a harder surface for armor plates or technical pieces use more hardener, if you want more organic shapes or flowing texture for things like faces and drapery use more softener. The mix ratio will also determine how quickly it cures. If you are working on something that is particularly challenging that you know you want to spend extra time on use a higher ratio of softener; this will make the putty cure slower and give you more time to work.
The main advantage of Brown Stuff is that when it hardens you can sand it or shave it with a scalpel. If you want to sand it use a fine grade sandpaper. This characteristic makes it best suited to hardline sculpting where you require a keen edge and flat surface. I often use Brown Stuff for things like armor plates, guns, and other technical parts. I normally recommend Brown Stuff to beginners because unlike say green stuff, which can't be sanded very easily, if you make an error with brown sometimes you can rescue a piece by going back and shaving out rough spots and uneven surfaces. This is something you can not do with green stuff. Brown also has a thicker consistency than green stuff so it is a little easier to work with for beginners.
Next time I will cover green stuff.