Mildew is a serious trouble. This is a vegetable growth and always a sure indication of dampness. It is impossible to satisfactorily paint a surface on which mildew has formed unless the surface is first treated to destroy this growth.

Ochre primers and ochre colors are particularly liable to this really serious trouble, due to the fact that they are largely of bog origin and contain the seeds or spores as they are called from which the mildew mold develops. Such growths result not only in a most serious discoloration of the work which at times may be taken as fading or change of color, but also are very destructive to the paint itself, mildew not only developing at times at the expense of the vegetable oil itself, but what is even more serious, growing between the wood and the paint and thus forcing the paint off.

Vegetable oils like linseed oil are not destructive to this vegetable growth, but turpentine is, hence the first thing to do in aggravated cases is to wash well and freely with turpentine, removing any loose paint; this will very largely destroy such growths. In addition, an exceptionally large amount of turpentine should be used in the first coat applied over such a surface; the paint should be well flatted. An undereoating well flatted with turpentine applied over a mildewed surface which has been washed with turpentine offers the best possible protection against repetition of the trouble.

Linseed oil is produced by expression from the seeds, either by hydraulic or steam power. This material varies in quality: According to the goodness of the seed from which it is expressed, and according to its age and clearness; for, when a large stock is kept, it is found that, in about six months, there is a considerable amount of accumulation of refuse at the bottom of the tank, which is only fit to be employed in mixing coarse paint for out-door work. The best is yellow, transparent, comparatively sweet-scented and has a flavor resembling that of the cucumber. Great consequence has been attributed to the cold drawing of this oil, but it is of little or no importance whether moderate heat be employed or not in expressing it. Several methods have been contrived for bleaching and purifying this oil so as to render it perfectly colorless and limpid, but these give it more beauty to the eye, in a liquid state, without giving it any permanent advantage, since there is not any known process for preventing the discoloring after its drying, and it is, perhaps, better upon the whole that this and every vehicle should possess that color at the time of using to which it subsequently tends, so that the painter may depend on the continuance of his tints, and avoid the disappointment and annoyance arising from a change of color.

Linseed oil is sometimes boiled with litharge to make it dry quickly, but when it is thus treated it is unfit for best work.

The quality of linseed oil may be determined in the following manner: Fill a phial with oil and hold it up to the light; if bad, it will appear opaque and turbid, its taste will be acid and its smell rancid. The oil which is expressed from good and full-grown seeds should, when held up to the light, appear clear, pale and bright; it is sweet to the taste and has little or no smell.

Linseed oil may be purified by the following process: Place the oil in a bottle or jar, and drop into it some powdered whiting, stir or shake up the mixture and allow it to stand on the stove, or in an oven, not too hot; the whiting will very soon carry down all color and impurity and form a precipitate at the bottom. The refined oil at the top may then be poured off.

In rare instances, where the least yellowness in the oil would be injurious, nut or poppy oil may be used with advantage; but, as already stated, linseed is the oil used for general purposes.

Oils of a nature suitable for painting are the most commodious and advantageous vehicle to colors hitherto discovered, first, because the unctuous consistence of them renders their being spread and layed on a surface with more evenness and expedition than any other kind of vehicle; secondly, because, when dry, they leave a strong gluten or tenacious body that holds the colors together, and defends them much more from the injuries either of the air or accidental violence than the vehicles formed of water. The principal and most general quality to be required in oils is their drying well, which, though it may be assisted by additions, is yet to be desired in the oil itself, as the effects of the pigments used in it are sometimes such as counteract the strongest driers, and occasion great delay and trouble from the work remaining wet for a great length of time, and frequently never becoming thoroughly hard. There are some oils that have this fault to an incurable degree. The next quality in oils is the limpidness, or approach to a colorless state, which is likewise very material; for where they partake of a brown or yellow color, such brown or yellow necessarily mixes itself with the pigments; but, besides the brown color which may be visible in the oil when it is used, a great increase of it is apt to appear some time afterwards when the oil is not good.

There are three changes which oils of the kind proper for painting are liable to suffer in their nature, and which affect them as vehicles, that are mentioned by painters under one term, that of fattening; notwithstanding, these several changes are brought about by very different means, and relate to very different properties in the oils.

The first is a coagulation by the mixture of the oil with some pigment improperly prepared. This, indeed, is called the fattening of the colors, but the real change is in the oils and the pigments are only the means of producing it. This change is generally a separation of the oil into two different substances, the one a viscid body which remains combined with the pigments, the other a thin fluid matter which divides itself from the color and thicker part.

This last appears in very various proportions under different circumstances, and, in some cases, it is not found where the pigments happen to be of a more earthy and alkaline nature, for then only a thick clammy substance that can scarcely be squeezed out of the bladder, if it is put up in one, is the result of the fattening. This fattening not only happens when oil and pigments are mixed together in bladders or vessels, but sometimes, after they have been laid on the proper ground for them, instead of drying, the separation will ensue, and one part of the oil will run off in small drops or streams, while the other will remain with the color, without showing the least tendency to dry.

The second is the change that takes place in oils from long keeping. This, if it could be afforded by the oil-manufacturer or the painter, is by far the best method of purifying linseed and other oils, as, by thus keeping, they become lighter colored and acquire a more unctuous consistence; and, though they are said to become too fat, they are in a very different state from that before mentioned, which is caused by unsuitable pigments.

The third is the change produced by artificial means, from exposing the oil a long time to the sun, whereby it is freed from its grosser and more feculent parts, and rendered colorless, and of a more thick and less fluid consistence than can be produced by any other treatment; but, at the same time, it is made less likely to dry, particularly when used with mineral colors, as vermilion, Prussian blue and King's yellow; it likewise becomes disqualified by other bad qualities that render it of little use as a vehicle for painting. Oils in this state are called also fat oils, though it is a change that has not the least affinity with either of the other, but, on the contrary, differs from both. In speaking, therefore, of the fattening of oils or colors, attention should be had to the not confounding these three several kinds one with the other.

Linseed oil, from its cheapness, is the only oil in common use for house painting, and it may, by proper management, be made to answer for every kind of work. This oil is pressed from the seed of flax, and is best when manufactured in great quantities. The general defect in linseed oil is its brown color, and its tardiness in drying, both of which are in a much greater degree found in some parcels than others. There is also found such as, in consequence of its being mixed with the oil of some other vegetable accidentally growing near it, partakes of the nature of olive-oil, and cannot be made to dry by any means whatever. The faults of the color and want of drying quality may be greatly reduced, if not entirely taken away, by keeping the oil for a length of time before it is used; it then becomes fat in the second sense of the word, as before explained, and is a good vehicle for color without any mixture; but it is generally used with a proper drier, as it never by itself becomes sufficiently pure to use with white or other light tints, without imparting a brown color to them.