To the elementary greens belong Green Ultramarine and the copper greens : Verdigris ; Schweinfurt Green and Scheele's Green, copper arsenites ; and Malachite Green, a native copper carbonate, which are no longer used, being poisonous.
(b) Composite Greens.—Chrome or Brunswick Greens, by far the most important class of greens in general use, are an intimate mixture of Chrome Yellow and Chinese Blue. They possess great brilliancy, good covering power, and cover a wide range of greens according to the relative proportions of their constituents, which determine their chemical stability and per manence. In oil these greens dry well and are darker and more glossy than in water. Being composite in their nature, they are more prone to colour changes, becoming yellower or bluer accord ing to the condition of exposure. Brunswick Greens prepared by grinding together the required proportions of Chinese Blue and Chrome Yellow show a tendency to "float," i.e., the blue rises to the surface of the paint ; this is largely obviated by the wet process of manufacture, the two colours being mixed in the striking or precipitating vat, washed and dried. Chrome Greens are extensively used as oil colours ; as size colours for paper ; in printing inks ; in textile printing ; and in the manufacture of lin oleum. Zinc Greens are similar to the Brunswick Greens being a mixture of Zinc Chrome and Chinese Blue. Though more permanent than Chrome Greens, they lack the body and covering power of the latter and on account of their higher price are not extensively employed. They are used as an artists' colour, for tinting enamels and in oil paints and printing inks.
Pigments.—Prussian Blues are complex alkali ferric ferrocyanides containing potassium, ammonium, or sodium according to preparation. The most valued blues are "potash" blues prepared from potassium ferrocyanide and ferrous sul phate, with subsequent oxidation. According to the method of oxidation, a number of Prussian blues are obtainable, varying in chemical composition and physical properties. These blues are of low opacity, but possess considerable covering power and are reasonably permanent, though readily destroyed by alkali. Prussian Blue is a dark reddish-blue with a red undertone. By special treatment, blues are produced possessing a high bronze lustre, known as Bronze Blues, which are much valued in print ing. Chinese Blue, the basis of Chrome Greens, is a paler blue than Prussian Blue and is characterized by a violet shade with a greenish undertone. Still lighter blues are the Milori and Steel Blues. Celestial Blue is a paler blue containing barium sulphate. Antwerp Blue is a Prussian Blue formed upon an alumina base. Prussian blues are soluble in oxalic acid; by treatment with this acid and subsequent salting out, Soluble Prussian Blues are ob tained. Other soluble blues are made by precipitating ferric chloride in excess of potassium ferrocyanide or by precipitating ferrous sulphate with potassium ferricyanide (Turnbull's Blue).
These blues are extensively used as oil and water colours and in spirit and oil varnishes, in printing inks, for staining paper, in textile printing and dyeing, and for toning blacks. Cobalt Blues are among the most permanent colours known, and exist in two forms: (I) Smalts, potassium cobaltous silicates of varying com position, King's Blue being richest in cobalt and Azure Blue darkest in colour; and (2) Cobalt Blue or Cobalt Ultramarine, cobaltous aluminate of variable composition ranging from a greenish-blue, which may contain combined zinc, to Thenard's Blue of reddish tone, prepared from alumina and cobalt phos phate. Coerulean Blue is a cobaltous stannate containing mag nesium. Somewhat restricted through high cost, they are used as artists' oil and water colours, in oil paints and enamels, for printing inks and in the ceramic industries.
Other blues in extensive use are the Ultramarines and a num ber of Organic Lakes. The copper blues : Egyptian Blue, Bremen Blue, Mineral Blue, etc., are practically obsolete.
Carbon Blacks form the main source of black pigments and have as their colouring principle amorphous carbon ; these blacks vary in colour and strength according to their origin. Vegetable Blacks are nearly pure carbon, obtained from the incomplete combustion of mineral oils, and are of a deep black shade giving a bluish-black tint on reduction. Lamp Blacks are brownish blacks of rather less intensity, being poorer in carbon, often giving as much as 25% of ash on ignition. Car bon Blacks, obtained chiefly from America by burning the natural gases, are very deep black in colour, denser and more granular than Vegetable Blacks, and are extensively used in the paint and printing-ink industries. Ivory Blacks were originally produced by charring ivory cuttings in closed vessels; these have been largely superseded by the Bone Blacks, which are bluish black in colour and are considerably denser and weaker than Carbon Blacks, containing only about o% of carbon admixed with calcium phosphate and calcium carbonate. Vine Blacks are prepared by carbonizing vine twigs and other non-resinous woods; they are beautiful bluish blacks with great depth of tone and are largely used in printing inks and are much valued as an artists' colour. Mineral Black is a dry grey shale containing about 3o% of carbonaceous and 70% silicious matter. It is also prepared from waste coal dust and is largely used in cheap black paints, often tinted with Vegetable or Carbon Black. Graphite is another variety of carbon of absolute permanence, used as a pigment for special purposes, e.g., in anti-rust paints and in the manufacture of "lead" pencils. Black Oxide of Iron is a triferric tetroxide,
which occurs in nature as the mineral magnetite. It is used as an oil and size colour and in the ceramic industry.