Most pigments sold in art/craft stores are intended to be used for painting. They are not made for marbling, and all need some management to become useful in the marbling process. To avoid some of this bother, purchase pigments made for marbling, by reputable marbling practitioners. Each of these latter products will work best used in the process specifically outlined by the seller.
I tried a lot of art store products, and now use Golden Fluid Acrylics with methylcellulose size. Golden has made available the best and most complete technical information about their products. I early on discovered that I prefer opaque, or nearly opaque, colors. I also wanted to use colors which were lightfast. Golden has color charts on-line which list an number of characteristics for each color, including specific gravity, lightfastness, list of all colors mixed in that specific bottle, opacity/transparency, to name a few. So first I scanned the list for colors which were most lightfast and opaque. Not surprising that most of these were what you might call "natural", or " historic" colors, made from earths, minerals or mined materials, and oxides. These are also colors which have a higher specific gravity. They tend to sink. Note: stuff can be added to to colors to make them float in a vat, but it is difficult to make "spreaders" stop. (Colors which spread ferociously are best used as the first color on the vat. They turn into veins as more colors are added. Or mix with a "heavy" color, eg titanium white) Through lots of experiments, I found that colors classed as "inorganic" were easier to use than "organic" colors, made from synthetic carbon compounds. Other color families which less troublesome and easier to use are the cadmiums, phthlocyanines, cobalts and pyrroles. Golden helpfully includes the family name of colors right on the bottle. Specific gravity is a guide to how much a paint will float. Specific gravity of 1 is the number for water. If the specific gravity of a paint is 1.20, it is an obnoxious floater. Most of the ones I find more manageable have specific gravities of 2.5, 3, 4, etc. Golden offers a handy chart of pigment density (specific gravity) of all their colors. Other reasons I like Golden Fluid: ingredients to help each color dry with the same surface reflectivity have been left out (the less ingredients in the paint, the better), high pigment load, all intermixable. They have very nice tech help available via telephone.
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