PIGMENT BASED INK PADS - These are usually glycerin based or some may be resin based. In general, all ink pads available today should be acid free and non toxic but it doesn't hurt to read the packaging to be sure!
Pigment ink pads have the special quality of being slow drying which makes them ideal for use when embossing!
Pigment inks tend to be thicker bodied than dye based inks and dry opaque with a vivid vibrancy and intensity of colour. Because the glycerin based dye is not absorbed, this intensity of colour is achieved with less ink than if it were dye based ink, as they 'sit' on the paper rather than being absorbed.
Pigment inks are suitable for use on porous paper and cardstock and work especially well when it is matte or embossed. It will also work on some non-porous/glossy material but usually only if embossing and being heat set. Sometimes, you may even wish to heat seat on porous paper.
Most metallic, pearlescent and interference colours tend to be pigment based and are great for embossing, however, for colours use the clear embossing pad.
Pigment based ink is a bit more difficult to remove from your stamps than dye based and will usually require the use of a rubber stamp cleaner.