Pigments for Decoration

Zinc White.

Oxide of zinc is a very useful pigment, being permanent in both oil and water. To no extent, however, does it rival, for general use, the oxides of lead, since it is vastly inferior in body—covering power—to genuine white lead.

Its principal merit is found in the fact of its be ing entirely free from the dangerous attributes, both during preparation and use, which attend the handling of white lead.

Pigments are very frequently adulterated, calcium sulphate or gypsum and silica being often used; for instance, gypsum in Venetian red and silica in ocher. White clay, or kaolin, is also very frequently used, but clay occurs naturally in such pigments as ocher, sienna, amber, Van dyke brown, and earths generally. Carbonate of lime or chalk is also used as an adulterant. It occurs naturally in some pigments, but only in very small quantities. It is often mixed with white lead. In some colors, such as rose-pink, chalk is used as a vehicle for carrying the color, and its use in these is therefore admissible. Barytes, when mixed with oil, is devoid of opacity and covering power; when mixed with pigments that are specifically lighter, the paint will separate after painting into two layers, the barytes being in the lower. Whiting and gypsum are better oil absorbents than barytes. Oxide of zinc, owing to its liability to chip off, will never replace white lead, but it may be used as a finishing coat to the latter.

Green Pigments.

The green most used is that known as chrome green. It consists of a mix ture of Prussian blue and chrome yellow, and is often prepared by precipitation of the two sub stances simultaneously from solution. The com mercial chrome greens consist usually of about one part color and three parts mineral white (generally barytes). They may conveniently be

divided into two classes—those inclining to yel low, and those inclining to blue in tint.

To tell whether any sample of paint is lead or zinc, put it into the flame of a gas-burner. If it is lead, it will turn brown by the action of the sulphuretted hydrogen of the gas; if it is zinc, it will keep white, and be unaffected.

To find out what substance any sample of lead is adulterated with would necessitate the use of a chemical laboratory, but a few simple tests may be of use to enable anyone to tell pure white lead.

White lead should not incline to pink or grey, nor be gritty. These are evidences of faults in the manufacture. There are sonic people who hold that barytes improves lead; but it is a ques tion whether 'this is so or not. A good way to test as to adulteration is to rub a portion of lead in a little sulphuric acid on a piece of glass with a bone palette-knife; if it is free from adultera tion, and a good lead, it will mix to a smooth paste. For a very strong and absolute test, take a piece of white lead, mix it up to the consistency of flatting with turpentine; let it stand till the lead settles, then pour off the turpentine, and add some benzine to it; then stir it well and shake it up, and let it settle again till next day. Then pour off the benzine, and place the white lead upon a piece of blotting-paper; you will then have your sample in a state of dry lead. Dissolve this in a little dilute nitric acid. If any sediment at all remains then, this will be the substance used for adulteration.