A good deal of misconception still exists in the decorating trade concerning the use, by paint manufacturers, of what are commonly known as " extenders" - a term which originated in America.
These may be described as inert substances, lacking in opacity, which are added to specific pigments for a variety of reasons. Barytes, asbestine, and whiting are among the substances in question.
It is true that extenders are almost invariably cheaper than the pigments to which they are added, and that in consequence their use means a saving of money to the manufacturer. Employed in moderation, however, they fulfil certain definite and valuable functions which vary according to the nature of the paint in which they are incorporated.
In some cases, they supply physical properties which are lacking in the main pigment or pigments. They may, for example, help to keep the pigment particles better in suspension in the medium, thus tending to prevent undue sediment. Again, they may possess the property of reducing gloss and can, for this reason, play a useful part in the preparation of flat paints and enamels.
Firms of repute use in the paints they make only sufficient extenders to fulfil certain requirements and, employed in this way, these substances must not be looked upon as adulterants, since they do not affect the durability of the paint, but endow it with properties it would not otherwise possess. Only when they are used in excess, with the sole idea of lowering the cost of the paint, irrespective of its behaviour in service, can they be considered harmful.