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Decorating Defects - their cause and cure

Internal causes of Discoloration.

So far we have outlined only discoloration of paintwork brought about by agencies outside the paint film ; we now have to consider the possibility of this defect occurring as the result of properties within the paint itself, but as this concerns the paint manufacturer rather than the painter, it need not be dealt with at any length.
The chemical properties of the great majority of ingredients used in paint making are thoroughly well understood and, in point of fact, darkening or fading of paintwork from this cause seldom takes place. There are, however, instances, the origins of which appear to be partly physical and partly chemical, and which, though rare, cannot altogether be ignored.
Darkening of Lithopone.-Lithopone pigments, for instance, possess what are described as " photogenic" properties which cause them, under the influence of sunlight, occasionally to assume a greyish appearance, ranging from pale lead colour to a dark slate, and to regain their original whiteness in the dark.
The cause of this phenomenon is not yet completely understood but it may be said that improved methods of manufacture in recent years-partly by the admixture of certain other pigments-have succeeded to a very large extent in overcoming this tendency which seldom makes its appearance nowadays. Many decorators have spent a lifetime in the trade without ever having had personal experience of it, and it is probably true to say that in most cases where it is suspected, the trouble is more likely to be due to dirt deposition.
Blackening of Vermilion.-Another instance of discoloration of paintwork which is far more rare to-day than it used to be is the blackening of vermilion, though the decline in its occurrence is mainly the result of this particular pigment having been largely superseded by the lake pigments known as permanent reds.
Vermilion consists of sulphide of mercury, which exists in two separate modifications, red sulphide and black sulphide. In certain conditions the red reverts to the black and the paint loses its brilliant red colour to assume a dark hue.
This change is more likely to take place when the pigment particles are unprotected, or only slightly protected by the medium. Hence, should it be necessary to employ it, the best safeguard is to use a colour-and­varnish finish, applying the varnish as soon as the vermilion coating is dry enough to receive it.
Floating Colours.-In certain conditions discoloration of paintwork may be caused by separation of the colour pigment from the white base, giving the illusion of fading having taken place. On rare occasions, paints made with Prussian blue may show colour changes, probably owing to the reduction of the blue in the absence of light. This pigment may sometimes show a slight tendency to " float" when the paint film is varnished, presumably owing to the extremely fine nature of the pigment particles, some of which detach themselves and rise to the surface. Similar changes of colour may conceivably take place with some of the red pigment lakes struck on a white base.
It should be emphasized, however, that though it is thought desirable to mention the possibility of discoloration from the causes referred to above, instances are extremely rare; thanks to the degree of control over materials and processes in modern paint manufacture they are not likely to be met with, save in exceptional circumstances.