As stated in the beginning of this chapter, the white pigment known under the collective term lithopone white, is becoming more and more appreciated, not only in the special industries referred to, but also by paint manufacturers and painters. That it required so long a time to bring this about was due to the many failures when it was being used for painting exterior surfaces without a thorough knowledge of its characteristics.
That it should not be employed in place of zinc oxide in admixture with white lead (lead carbonate) and that driers with lead compounds should not be used when being mixed for spreading was well understood by chemists and most all paint manufacturers, but to the general consumer this information was a sealed book. Many of the so-called combination leads or combination whites had this zinc sulphide white as their base, because of its comparatively lesser cost and greater hiding power as against zinc oxide, and wherever discoloration or blackening of white surfaces resulted on exterior, where such combination whites were used, misleading explanations were usually made. This is now becoming better understood and paint manufacturers will confine the use of lithopone to colored paints for exteriors and so far as clear white is concerned to interior painting material. The tendency of lithopone to become gray when exposed to the direct rays of the sun has caused many a sleepless night to manufacturing chemists engaged in the production of lithopone and, while the subject is becoming better understood from year to year, and while many claim that they have discovered the cause and a remedy for the trouble, it crops out every once in a while unexpectedly with the very material for which a freedom from the effects of sunlight is claimed. When lithopone is mixed with water to a paste and applied to a strip of glass or other surface and immediately exposed to direct sunlight, the white assumes as the water evaporates a gray color, sometimes it becomes nearly black. When the strip is removed to a dark place the material becomes nearly if not quite white again. This property of absorbing light and giving it out again is also noticed when lithopone is ground in and thinned with linseed oil or varnish in neither of which any lead driers have been used. When such white paint is applied on an exposed surface where the direct rays of the sun strike it before it has had an opportunity to dry thoroughly hard it will turn gray readily and, while it sometimes will regain its natural whiteness, such is not always the case. The reason for this has never been fully explained, but the writer has tried out numerous samples of imported and domestic lithopone, all being of approximately similar composition in the percentages of zinc sulphide, zinc oxide and barium sulphate. Every sample was mixed with the same vehicle, damar varnish from the same package in like proportions and applied side by side on a strip of wood, previously coated with zinc oxide paint and exposed at the same time to the south during the day. No remarkable change occurred until next morning, after the sun had been out for an hour, when every one of the samples had become discolored to a greater or less extent, two of the ten samples being nearly black, six having assumed the gray of agateware, while the other two showed a very light lead color tint. On examination it was found that none of the samples were dry, all showing the same decided tack. There had been a slight precipitation of dew during the night, which may account for the result, as it is a fact, that the presence of moisture in or on lithopone paint will aid in its discoloration by sunlight. The singular phenomenon in this test was, that after two days and nights, when the damar varnish had dried hard, all but the two darkest of the samples had regained their whiteness, while these two had assumed a dark cream tint. Whether the acidity of the varnish assisted in the discoloration of the samples the writer does not care to say, but he is quite certain that lithopone, when mixed with gloss oil, a solution of ordinary pine rosin and petroleum naptha, and applied to an exposed surface, as on the head of a barrel, standing in sunlight, will turn a dark gray in a few minutes without the presence of moisture. Still, that moisture will have a disastrous effect on paint made with lithopone when exposed has been proved by the writer, who tested out paints with lithopone as the base, thinned with linseed oil and manganese drier, to which various percentages of a watery emulsion was added. Even though the paints were tinted, the results were astonishing, the greatest change showing in the tests made with the paint having the greatest percentage of emulsion.
There are many engaged in the manufacture of lithopone here and abroad who claim that they have perfected their processes so as to overcome all the objections to its use on exteriors and even have obtained patents to protect their inventions, but it is wise to be skeptical and make use of this wonderful pigment where it is known to be safe. The results shown by the panels painted with lithopone on the Atlantic City test fence on which a very high grade of lithopone was used, seem to prove that so far lithopone, because of its lack of durability, not to speak of discoloration (which was not seriously considered) is not to be considered as a good exterior paint, at least not, when thinned in the same manner as lead and zinc paint. Summing up its advantages and prospects, we may say, that it has come to stay and that a great future is before it, if not in the general line of exterior painting and decorating, it will be for interior work and in industrial consumption, which will increase from year to year as the country grows still more and more. As stated before, it has within the last few years, made great strides in the manufacture of interior flat wall paint, where it is more sanitary than white lead paint, flatting far better than zinc white paint, and being more durable than cold water paint, at the same time being moderate in cost. It has become indispensable with oilcloth and shade cloth makers, in the rubber making industry and to many paint manufacturers in the production of floor paints, ship paints and cheap grades of enamel.
And it is in this pigment that the much-abused mineral, barytes or barium sulphate, has found a place where it has rehabilitated itself as the very useful adjunct to paints, that it really is and always has been. Here is, where it does not serve simply as a make weight or extender, but as a helpmate.