Januray- Composite Paints- A S Jennings

Januray- Composite Paints- A S Jennings

A selection of historical paint references

By Peter Walters

My dictionary defines “Composites” as
1. A structure or an entity made up of distinct components.
2. A complex material, such as wood or fibreglass, in which two or more distinct, structurally complementary substances, especially metals, ceramics, glasses, and polymers, combine to produce structural or functional properties not present in any individual component.

In our industry the word has come to mean those engineered materials which we encounter as substrates for painting, which consist of a heterogeneous mixture of components, such as GRP or fibre cement board, in keeping with the second definition of Composite above.

For this Brush Strokes theme of composites I found an extract in my most popular source of material for Painted Memories, the 1935 three volume work by A S Jennings entitled "The Modern Painter and Decorator" in which the term 'composite paints' is interpreted in a way more consistent the first definition of composite from my dictionary. Arthur Seymour Jennings defines 'composite paints' as a paint in which more than one pigment is used.

In the days when the painter made his own paint by dispersing pigment, usually white lead, into raw or boiled linseed oil, and reduced the paint to application viscosity with turpentine, a composite paint was usually one produced by unscrupulous tradesmen in which the cost had been reduced, and quality compromised, by adding extenders to the mix.

A S Jennings, however, argued that 'modern composite paints' could, by the mixing of two or more prime pigments and extenders, produce a paint superior to that obtained by using a single 'pure' pigment.

The extract for this edition of Painted Memories is taken from pages 121 to 124 of Volume 1 of the Modern Painter & Decorator - Chapter 5 General House Painting - Zinc Oxide Paints - Composite Paints.

In former years the prevailing opinion was that paints made from a single pigment were the most durable, and white lead was the favourite in this respect.

Some twenty years ago in the United States of America the paint manufacturers formed a scientific association and contributed considerable sums of money to investigate the subject of composite paints. With this object they caused to be painted various panels of wood, iron, etc., and had them exposed in different parts of the country to the weather. These panels were examined periodically by a small committee of expert painters, and their condition reported upon. The result indicated that a paint made up of several pigments was more durable than any one used by itself. The best illustration can probably be given of this fact in the case of white lead as compared with zinc oxide. The former may be looked upon as being an excellent paint in many respects, but one which is liable to chalk - that is, after a time the oil with which it is necessary to mix becomes destroyed, and the paint is reduced more or less to a condition of whitewash. This is particularly noticeable when paints are used at the sea-shore, and the test for chalking is to rub a piece of black cloth over the surface, when part of the decayed paint will come off. Zinc oxide, on the other hand, has the tendency to be too brittle and to crack. The mixture of the two, therefore, in proper proportion has this effect - one corrects the shortcomings of the other.

There is a further point to be taken into consideration, which is, providing for subsequent painting. If the paint at the end of a period, say that of three, four, or five years, is not in a condition to receive a new coat of paint, then the expense may be largely increased.In this respect, a paint which fails by chalking obviously possesses great advantages over one which breaks down by cracking. In the former event, only a thorough rubbing down will be necessary before repainting, whereas if the cracking which has taken place extends to any depth, it will probably be essential to burn off before any repainting is carried out.

Most of the ready-mixed paints on the market are made from several pigments ground in oil and turpentine. Even graphite paint can be mixed with other materials with advantage, particularly in respect to improving its appearance, graphite being somewhat sombre.

It will be understood, from what has been said, that the question of purity in paint is a misnomer. One can call "pure," pure white lead, pure zinc oxide, but this does not necessarily mean that, if used alone, they make the most durable paint. Some years ago certain American States passed laws for rendering it compulsory for paint manufacturers to disclose the formulae of all paints sold, on the label attached to the packet. This certainly had the effect of stopping the sale of impure ready-mixed paints, which often contain such materials as whiting or carbonate of lime, which should never be used in oil paints. But it did little in remedying the evil, and was a gross injustice on the manufacturers in giving away special formulae which may have been discovered after considerable research. As far as staining colours are concerned, the question becomes more important, because the addition of white cannot assist in any way the staining or colouring properties, and can only be put in the colour for the purpose of cheapening it. There are, however, certain exceptions, notably in the case of Brunswick green and some of the lakes which must be precipitated on a white base. In these cases the presence of white, such as barytes, cannot be considered an adulteration.

The following remarks by the well-known paint-expert, Mr. A. H. Sabin, on composite paints will be read with interest. The pigments come under two classes, the 'base' and the 'tint.' Both these terms are highly descriptive. The base pigments give to the paint its 'body,' or consistency. On the proper selection of the base pigments depends to a very large extent the quality of the paint, whether considered from the point of view of its application by the painter, or its endurance and protective effect on the surface to which it is applied.

The 'hiding power,' or capacity to conceal the surface over which the paint is applied, the spreading rate or the area of surface over which it will extend in a continuous layer when applied with a brush, the protective power, or power to protect the surface from atmospheric agencies of destruction; these and many other valuable properties are to a very large extent controlled by the base pigments.

Bearing these principles in mind, let us now consider the first specification, 'woodwork exterior' paint, in detail, beginning with the first formula.

Alternative formulae per cent.

Zinc Oxide45 50 50 50 50
Basic carbonate white lead 20 20 --- 35 50
Basic sulphate white lead25 20 35 --- ---
Asbestine, or silaex, or barium sulphate 10 10 15 15

We have here four pigments: zinc oxide, basic carbonate white lead, basic sulphate white lead, and a choice of three others, namely, asbestine, or silex, or barium sulphate. Zinc oxide is one of the best individual white pigments. In whiteness, hiding power, permanence, durability, inertness to oil and protective power it takes front rank. When exposed to air containing sulphur compounds which would darken or blacken white lead, it remains white, as the sulphide of zinc is as white as the oxide. It does not chalk or crumble, and it does not become transparent.

Then if this is the case, why not use zinc oxide alone in our paint? Why all, the other constituents? The answer is that zinc oxide, when used alone, has certain economic drawbacks which vanish entirely when it is used in a properly compounded mixed paint.

In the first place, owing to its excessive fineness, zinc oxide forms a perfect blend with the oil, and in consequence spreads out under the brush to an exceedingly thin layer. Hence, in order to get the thickness of paint coating necessary to cover and protect properly, more coats of paint are required than, say, with white lead. Now when we mix white lead and one or more of the three pigments, asbestine, silex, or barium sulphate, with the zinc oxide, we stop this tendency to excessive spreading. These pigments are, comparatively speaking, coarse-grained. They have what painters call 'tooth.' They take hold of the surface of the wood and cling there, and prevent the paint from spreading out too thin, and the necessary covering can be got with fewer coats of paint.

Secondly, a paint consisting of zinc oxide and linseed oil only dries with a hard smooth surface. The advantages of this we shall deal with later. The disadvantage is that when, in the course of years, the oil becomes dry and decomposed (as happens sooner or later with all paints) the paint coat cracks and peels off in patches. This makes it impossible to do a good job of repainting unless the old paint is burnt off or otherwise removed. Now white lead does not do this. When a white lead paint fails, it does so by crumbling to powder, or, as painters say, 'chalking.' The surface of a chalking paint gives an even surface for repainting.

Now when we make up a paint composed of zinc oxide, white lead, and inert pigments, we get the advantages proper to each of the pigments. The zinc oxide imparts whiteness, durability, and permanence of colour; the inerts and white lead enable the painter to get a good covering layer of paint for each coat he applies, and the white lead ensures that a good surface for repainting shall be present when in course of time the paint has perished. And zinc oxide does one thing more. If we paint two boards, one with zinc oxide and one with white lead, and expose them to a dusty atmosphere, the white lead board will accumulate dust much faster than the zinc oxide board. The smooth surface of the latter makes it very difficult for the particles of dirt to adhere, whilst the comparatively rough surface of the former holds them. This is particularly well shown when a wet sponge is passed over the boards. The zinc board will show up clean and white after this treatment, whilst the lead board will remain dirty. A board painted with a mixed paint such as the specifications show behaves in this respect like a board painted with straight zinc oxide. The zinc oxide, owing to its excessively fine state of division, blends so thoroughly with the oil that it enters the glossy surface which the oil forms over the paint, so that the surface of the paint coat has the same dirt-repelling - and we may say also wet-repelling - power that a straight zinc oxide paint possesses, and at the same time the mixture is free from the tendency to crack and peel and gives a good surface for repainting. The effect of this property on the protective value of the paint is obvious.

Hence in the specification paints we have a combination of the advantages which the different pigments possess, so blended that the pigments. reinforce .one another, and make, practically speaking, a better paint than anyone of these could make alone.

As regards the two white leads, basic carbonate white lead and basic sulphate white lead, both are useful as ensuring a good repainting surface. Basic sulphate white lead is a very fine-grained pigment, and paints made from a mixture of zinc oxide and basic sulphate white lead require the presence of one of the coarser pigments to regulate the spreading.

When basic carbonate white lead is used, this is not necessary, as this pigment supplies the necessary tooth.
It will be noticed that 15 per cent is specified for 'inert' pigments. These are too transparent and too coarse-grained to be used in large amount, but in moderate percentages are useful as conferring tooth.