This section is from the book "Chromatography; Or, A Treatise On Colours And Pigments, And Of Their Powers In Painting", by George Field. Also available from Amazon: Chromatography, or A Treatise on Colours and Pigments, and of Their Powers in Painting.
White Lac Varnish is a new varnish, prepared by dissolving in alcohol the lac resin of India deprived of all colouring matter, and purified from gluten, wax, and other extraneous substances with which it is naturally combined; without which process the varnish it affords is opaque and of the dark colours of the japans and lacquers of the East, but when thus purified, its varnish is brilliant, transparent, and nearly colourless. This varnish being of recent introduction, has hitherto been only partially used in painting; being also a spirit-varnish that requires a warm temperature, that dries rapidly, and combines reluctantly with oils, it is difficultly manageable as a vehicle. It has, however, been employed, by rubbing, into oil colours on the palette, with a view to the giving them permanence by clothing them, and a crisp-ness which makes them stand up and keep their place. Its extreme transparency, and the power with which it causes colours to bear out, have occasioned it also to be used in the progress of a picture, to bring out and preserve the force and richness of deep colouring and shadows; and also for varnishing over those parts during the painting which dry difficultly, so as to proceed immediately with the work: a coat of this varnish interposed will also effectually prevent any injurious action between pigments which cannot otherwise be employed together with safety, or which do not cordially unite. Its use in forming a medium, or process intermediate to oil and water painting, has already been pointed out and unfolded. To what more extensive use the ability and ingenuity of artists may apply it in oil-painting remains to be investigated.
The principal recommendations of white lac, as a varnish, are the remarkable power and effect with which it brings out the colouring and design of a picture, and the permanence with which it so preserves them; it being neither subject to bloom, chill, nor crack, when properly applied, according to the general rules for all varnishes, and those of spirit varnish in particular; the principal of which are a dry atmosphere and summer warmth of the apartment in which they are used At a temperature of not less than 60° it dries in a second or two, and coat after coat, as it dries, may be applied with a broad soft camel-hair brush. This soon becomes harder and firmer than any other varnish, and entirely free from the tackines with which they catcli and retain the dust and floatings of the atmosphere, and from the opacity and discolourment by which varnishes ultimately obscure pictures on which they are applied; so that removing this varnish from a picture becomes an unnecessary operation, though this may be easily effected, if required, by the proper use of spirit of wine: nor has any instance occurred of its cracking in the variety of cases in which it has hitherto been employed in the painting and varnishing of pictures, however freely it may have been used. This and all other spirit varnishes differ from those of fixed or expressed oils in this respect, that spirit and essential oil varnishes are dissoluble, and removable from pictures, by alcohol and essential oils, etc.; but fixed oil varnishes are insoluble by such means, or any other not injurious to the painting itself.
All drying oils, in proportion to their power of drying, are disposed to acquire colour by age, and all resins, in proportion to their softness, have the same propensity; both these in time, therefore, obscure the pictures on which they are employed. Oils are, nevertheless, long preserved from this change, either by varnishing before they are much dried, or by constant exposure to light and air, and the white lac varnish is especially valuable for this purpose, by a thin coat of which over a recent painting, its firmness and colour are secured; after which a soft fat varnish may, if required, be applied over the lac, and may be easily removed by friction, or oil of turpentine when discoloured, without displacing the lac or injuring the thinnest glazing.
Lac varnish may be combined with mastic varnish, in small proportion, so as greatly to improve it; it may also be employed in the manner of other spirit varnishes in varnishing drawings, prints, etc, which have been previously sized with isinglass; and, being first thickened by setting it to evaporate in an open vessel in a warm place, it may be passed over miniatures, etc. without previous sizing, so as to give them much of the strength, force, and durability of oil-paintings: but in all these cases it should be used in warm dry weather, or near a fire.
General Remarks. - Upon comparing the qualities of the varnishes of mastic, copal, and lac, it will appear that the latter are successively harder and more perfect as varnishes, and in proportion to their perfection as varnishes is the difficulty of using them as vehicles; and as it is necessary that before varnishing with any of them the picture should be thoroughly dry, to prevent subsequent cracking, this is perhaps more essential for the latter than for the former. Notwithstanding this necessity, there is one highly important advantage which seems to attend early varnishing; namely, that of preserving the colour of the vehicle used from changing, which it is observed to do when a permanent varnish is passed over colours and tints newly laid; but this it does always at the hazard, and often at the expense, of cracking, and early varnishing with soft varnish dries slowly and is more disposed to bloom.
This saving grace of early varnishing appears to arise from the circumstance that, while linseed and other oils are in progress of drying, they attract oxygen, by the power of which they entirely lose their colour; but, after becoming dry. they progressively acquire colour. It is at the mediate period between oils thus losing and acquiring colour, which commences previously to the oil becoming perfectly dry, that varnish preserves the colour of the vehicle, probably by preventing its farther drying and oxidation, which latter may in the end amount to that degree which constitutes combustion and produces colour: - indeed it is an established fact, that oils attract oxygen so powerfully as in many cases to have produced spontaneous combustions and destructive fires.
It is eminently conducive to good varnishing, in all cases, that it should be performed in fair weather, whatever varnish may be employed; and that a current of cold or damp air, which chills and blooms them, should be avoided. To escape the perplexities of varnishing, some have rejected it altogether, contenting themselves with oiling-out - a practice which, by avoiding an extreme, runs to its opposite, and subjects the work to ultimate irrecoverable dulness and obscurity.
Blooming is the defect chiefly of soft resins, such as mastic, etc, and the more recently they have been applied, the more disposed they are to this annoyance. Now there is always free oxygen enough in light and air to deprive a very thin film of drying-oil of its colour; and, as oil is not subject to bloom, if a varnish have oil in its composition, its blooming will be corrected: hence some artists have varnished their pictures with macgilp, but in escaping thereby the evil of blooming, they have run into the greater evil of the ultimate discolourment and irreparable obscurity of their work; and, if a varnish have already bloomed, the slightest portion of oil rubbed over the surface of a picture and immediately polished off with an old silk handkerchief, will remove and prevent it - repeatedly if the blooming should reoccur.
The manufacturing processes of the varnishes now generally used have been detailed in the Transactions of the Society of Arts, etc, Vol. XL1X. But with regard to the recipes for compounding varnishes, etc, superabounding in antient and modern treatises, however flatteringly recommended, there are few elegible or of practical utility, and yet fewer justifiable to art and good chemistry by the simplicity upon which certainty of effect depends, being in general quite of the class of recipes and formulae of the old cookery books and dispensatories.