Dyes, Pigments, and Colour Preparation
What the exam tests
This domain tests dye classes, pigment systems, mordants, thickeners, and the practical preparation of printing paste. Expect questions on paste viscosity, compatibility, shade matching, and water quality.
Core rules
- Dye classes behave differently: reactive dyes bond to cellulose; vat dyes need reduction and oxidation; indigo is a vat dye; azoic dyes form colour on fibre; pigments are insoluble and need a binder.
- Mordants (alum, tannin, iron) help some natural dyes and certain dye classes fix; excess iron can dull shades.
- Thickeners give the paste body: sodium alginate for reactive dyes, guar gum or starch for many pigment and discharge pastes. The thickener must suit the dye chemistry.
- Paste viscosity should let the block pick up an even film without running; too thin bleeds, too thick gives incomplete coverage.
- Mix dye powder into a paste first, then dilute; strain to remove lumps.
- Use clean, low-hardness water; hard water can precipitate dyes and shift shades.
Worked example
A reactive paste is made with starch thickener and prints dull, unevenly fixed colour. Reasoning: starch is not the preferred thickener for reactive dyes because it can react or hinder penetration; sodium alginate is more suitable. The fix is to reformulate, not simply add more dye.
Traps
Do not mix incompatible thickener and dye. Do not judge shade from wet paste alone. Do not ignore water hardness or pH; both change reactive dye results.