Читать книгу The Mixing of Colours and Paints - F. N. Vanderwalker - Страница 32

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These are quick drying varnishes bought in ready prepared form or mixed by decorators by adding bronze powder or other colors to clear wood lacquer.

Some of the poor grades of yellow ochre retard the drying of a paint film. They are inclined to retain moisture, and that characteristic promotes the growth of mildew in certain climates. The popularity of yellow ochre twenty-five or thirty years ago was eclipsed by the use of cheap grades of yellow ochres, because they contained moisture and did not anchor firmly in the pores of the wood. Much cracking and scaling of paints resulted. The use of real French ochre did not result in this condition, but the cost of this high-grade color was greater than the cheap ochres, and so it was not extensively used.

Raw umber has a rather large content of manganese, which makes it an excellent drying color; in fact, raw umber is used sometimes in the manufacture of liquid driers.

Lampblacks are made in so many different ways that it is difficult to determine upon examination just what quality a package contains. In the burning of oils or other substances to make lampblack, the introduction of too much air produces grayish blacks, and too little air results in a content of oil or volatile acid. When the burning process is forced too much, the resulting product is not only an inferior black, but one which contains both oil and acid in sufficient quantities to cause spontaneous combustion. This is a dangerous dry pigment to handle; it is slow-drying and likely to corrode metal surfaces because of the acid content. In other words, black paint made from such lampblack is apt to destroy metal surfaces which it is supposed to protect.

Gas and carbon blacks, which are sometimes added to good lampblack to increase tinting strength, are detrimental, not alone because they change jet blacks to brownish tones, but because they result in a separation of the pigment and the oil. Uneven, streaky black paint and muddy gray tints are also caused by these inferior gas and carbon blacks.