_Ultramarine_--A substance of complex composition prepared by heating a mixture of china clay, carbonate of soda, sulphate of soda, sulphur, charcoal, and sometimes quartz, rosin and infusorial earth. _Prussian Blue_--A compound prepared by adding potassium ferrocyanide to a solution of ferrous sulphate. _Brown._--Natural earth colours, such as sienna, umber, Vandyke brown. _Black._--Lamp-black, bone-black, Frankfort black. (B) Produced by the reaction of soluble salts upon one another when added to the pulp in the beating engine. _Yellow._--_Chrome Yellow_--The paper pulp is first impregnated with acetate of lead, and potassium or sodium bichromate added. This precipitates the chromate of lead as a yellow pigment. _Chrome Orange_--The addition of caustic alkali to the bichromate solution converts the chrome yellow into an orange. _Blue.--Prussian Blue_--The paper pulp impregnated with iron salts is treated with potassium ferrocyanide. The blue colour is at once obtained. _Brown.--Iron Buff_--A light yellow-brown colour due to the precipitation of ferrous sulphate by means of an alkali. _Bronze._--Manganese chloride followed by caustic soda. SOLUBLE COLOURS. (A) Natural Dyes. These colouring matters are now seldom used. _Yellow and Brown._--The vegetable extracts, such as fustic, quercitron, cutch, turmeric, have practically all been replaced by aniline colours. _Red._--Madder (Turkey red), Brazilwood, cochineal (a dye obtained from dried cochineal insects). Safflower. _Black_.--Logwood, used in conjunction with an iron salt. Cutch, used with an iron salt. (B) Coal Tar Dyes. The dyeing and colouring of paper pulp by means of the artificial organic substances has become a matter of daily routine, the expensive natural dyes and the ordinary pigments having been almost completely superseded. The numerous colouring matters available may be classified either by reference to their chemical constitution or simply on general lines, having regard to certain broad distinctions. If the latter classification is taken, then the dyes familiar to the paper-maker may be divided into-- (a) Acid dyes, so called because the full effect of the colouring matter is best obtained in a bath showing an acid reaction. (b) Basic dyes, so called because the colour is best developed in an alkaline solution, without any excess of mordant. (c) Substantive dyes, which do not require the use of a mordant, as the colour is fixed by the fibre without such reagents. Some of the most frequently used colouring matters are shown in the accompanying table on page 202.