Varnishes And Oleo-Resinous Media
( Originally Published 1913 )
ONE of the oldest kinds of varnish was the oleo-resinous vehicle known under the name of " Vernice Liquida," and was probably of the same nature as that used on the oldest, as well as the more modern, tempera pictures as a finishing coating to protect the tempera colours. This varnish is mentioned in most of the old manuscripts that treat of the technical processes of painting.
The term "vernice liquida " frequently occurs in the MSS. of Alcherius (1382-1411), of St. Audemar, often in Cennini's treatise, in the Venetian MS., in the Notes of Le Begue, and Vasari also mentions this varnish in his account of the life of the painter Alesso Baldovinetti.
This old liquid varnish simply meant " varnish," as the latter term in the modern sense of the word implies any kind of an oleo-resinous composition in a liquid state that is used in painting and that dries with a lustrous appearance, but formerly the old word " vernice," or " vernix," was the name given to one of the ingredients of the composition, namely, the gum or resin in its dry state. The liquid of the vernice liquida was either a drying or " fixed " oil, such as linseed or walnut oil, or in rarer instances an essential oil, such as the oil of spike, or spirits of turpentine, and in some cases it was a mixture of a drying and an essential oil. In many early documents that give recipes for the making of vernice liquida the word " vernix " was often used to designate the dry sandarac resin, one of its ingredients. Sandarac was also a name given by the Greeks to a certain red pigment, possibly red orpiment, from its re-semblance to the red colour of sandarac resin which the latter acquires with age. The word sandarac is derived from the Persian sandarus, which means a red colour. Amber resin, a more valuable substance, was also employed in the making of varnish, and in some old documents it has been confounded with sandarac.
The sandarac resin is the product of the Thuja articulata, the African Arbor vitæ, a kind of dwarf pine, resembling the juniper tree. It is a native of Morocco, and is also found in some other countries of the East. The fresh sandarac gum or. resin is yellow in colour, but gradually grows red with age, and also darkens to a reddish colour when heated with oil in the process of varnish making. We have mentioned that some early authorities speak of sandarac resin and dry vernix as one and the same substance ; they also in some instances have mistaken the Thuja articulata for the juniper tree, as will be seen from the following extracts quoted by Eastlake from writings on medicinal subjects of the sixteenth and seventeenth centuries, for example : " The juice which flows from the juniper is called vernix." " From dry vernix and linseed oil, liquid vernix is made." " The juniper produces a resin similar to mastic, called sandarac. This, when fresh, is light in colour and transparent, but as it acquires age it becomes red." " With this resin and linseed oil is prepared the liquid vernix, which is used for giving lustre to (tempera) pictures, and for varnishing iron." Again : " It is prepared from the sandarac of the Arabs ; this is called dry vernix. From this and linseed oil is made the dark (red) liquid varnish so well adapted for giving lustre to pictures and statues," etc. Also : " Vernisium, the same as vernix, otherwise called sandarac, or juniper resin, and thus dry vernix, also the fluid composition prepared from the resin, then, or formerly, called liquid varnish." The latter extract is from a book on medicine by B. Costello, Geneva, 1746.
The general and modern meaning of the word " vernice or varnish, prevailed from about the beginning of the seventeenth century, when the term " liquid varnish " became obsolete, and sandarac was no longer a synonym for (dry) vernix.
The old formula for making the common kind of vernice liquida of a moderate consistency was, one part of sandarac resin to three parts of linseed oil. The oil was heated to a boiling point, and the resin was added in small quantities at a time to the boiling oil and kept well stirred until the whole of the resin had been finally dissolved and thoroughly incorporated in the mixture, which was then allowed to cool. A superior sort of this varnish was known as " vernice liquida gentile," and was made by dissolving one part by weight of yellow amber in three parts of boiling linseed oil. This would make a stronger and more elastic varnish than the commoner kind made from sandarac. According to Tacitus and Pliny the old German name for amber was " glassum " (glas), signifying perhaps its clear and transparent nature. The terms " glas " and " glassa " were often applied to amber, and in some old documents to sandarac as well. The term " verenice " (vernix) was used by the Greeks to signify amber, and this word was often used in later times, though in-correctly, as a designation for sandarac.
Vernice liquida prepared from sandarac was usually reddish and dark in colour, and as such was preferred by the Byzantine painters, whose pictures always show that a dark varnish must have been applied over the tempera colours. The early Italian painters purposely executed their work in a light scheme of colouring, relying on the dark varnish to give their pictures the necessary rich and darkened tones. In these cases the old painters used their dark varnish as a lacquer, and some went so far as to tint the varnish with transparent pigments in order to make a lacquer to glaze over their pictures, to give the colours more depth, and so enhance their brilliancy and richness. Red sandarac varnish was also used to counteract the greenish tones of the under-painting of the flesh tints by the early Italian painters.
On the other hand, occasions arose when paintings, or certain parts of them, should be kept light and brilliant, and for such parts a white, or some pale variety of varnish, sometimes mentioned as " chiara," in old documents, was sought for. This was provided by the white varnish made from concrete turpentine, and sometimes from mastic, or even from a mixture of one or both with sandarac and linseed oil.
Turpentine resin in the concrete state was called " white resin," or " fir resin," and some-times " glorie," or " gloriat," from the gloss it imparted to varnish. The Greek names for it were " pece " and " pegola." In some documents of the sixteenth century concrete turpentine is often mentioned under the name of " pece Greca." This substance is obtained from many trees of the Coniferous order, such as pine, spruce, and fir trees. Common resin, or colophony, is the solid residue obtained from concrete turpentine after the oil is distilled from it.
It was found that turpentine resin when mixed with the other ingredients of varnish increased its gloss or brilliance, and for this reason the varnishes which contained it were in common use for the varnishing of implements and furniture, as well as for pictures.
Varnishes composed of resins and " fixed," drying oils, dry, like the oils they contain, by what chemists call " oxidation," that is, in the process of drying they take up oxygen from the atmosphere which unites with the oil and forms a sort of waterproof pellicle, or skin, on the surface, not unlike india-rubber in texture ; while on the other hand, varnishes made from resins or gums and essential oils or spirits, such as oil of turpentine, oil of spike, naphtha, alcohol, etc., dry, like the oils of their composition, by " evaporation."
When varnishes that dry by oxidation are mixed with badly drying pigments, or when spread over the latter before they are thoroughly dry, the surface will likely become wrinkled or cracked, for in such cases the upper surface will oxidize, or dry, much sooner than the underlying pigment, and in course of the drying of the colour underneath it will cause wrinkles, or tear the upper surface sooner or later and produce cracks.
The ordinary mastic varnish in use at the present day is an essential-oil varnish composed of gum mastic and oil of turpentine. It is the best of all spirit varnishes, as it does not darken colours, and is not liable to cause wrinkling or cracking of picture surfaces. The fixed-drying oil varnishes used in painting embrace copal, amber, kauri, sandarac varnishes, etc., and their drying power is hastened, not only by the boiling of the oil in the process of manufacture, but by the addition of one or other of such substances as sugar of lead, acetate of lead, litharge, manganese, and white copperas.
When the Van Eycks sought for a varnish to apply to their pictures, one that would dry in the shade, they evidently, after many experiments, discovered what they sought for, namely, an oil varnish to which they probably added some of the above-named drying ingredients. Such a varnish would dry almost as well out of the sun as in it because of its drying by oxidation. We may infer from this that the varnish formerly used by the Van Eycks, and other painters before their time, was either an essential-oil varnish, or one very like such a varnish in composition, that only dried in the sun, or by heat—that is, that its drying, if not due altogether to evaporation, was at least hastened by heat of some kind. It appears, however, that having found an oleo-resinous varnish to suit their purpose the Van Eycks used this as a vehicle for their colours, and apparently ground their pigments in this vehicle, instead of water, as formerly, and found them to appear more lustrous and richer by doing so. They also found that their pictures when painted with colours that had been ground in the oleo-resinous vehicle, did not require a final varnishing, like that which had been given to paintings which had been executed in the older tempera methods, as the colours in the new method dried with a gloss, and were protected enough, without the addition of a coat of varnish.
It may be suggested here that the oleo-resinous mediums used by the Van Eycks and the Flemish painters who immediately followed them consisted of mixtures composed of the concrete turpentine resin, damar, or mastic gums, dissolved by hot turpentine oil, to which a small quantity of linseed or walnut oil was added in the making to counter-act the otherwise brittle quality, and to give toughness or elasticity to the varnish. The clear and brilliant state of the colours in the pictures by the Van Eycks and other early Flemish painters would suggest the use of a medium made from balsams or other white, or very light coloured resins, also, the pece greca, or fir-resin, would protect the colours from damp or moisture more effectually than even amber or copal, and so preserve their lustre. If the latter resins or gums were used as ingredients of some of the early Flemish varnishes, such varnishes were only used on the darker coloured portions of their pictures. It has been suggested that the Van Eycks ground their colour in walnut or linseed oil, and used the varnish mixture of their invention as a painting vehicle or diluent, but there is no positive evidence of this.
We may be pretty certain that the new oleo-resinous vehicle discovered by the Van Eycks was a freely working medium, and not so sticky or glutinous as our modern oil varnishes, or if it was so, when freshly made, it must have been diluted with some thin diluent like oil of turpentine, oil of spike, naphtha, or perhaps with a thin and slow drying fixed oil like poppy seed oil or walnut oil. In any case it must have been a thinner and a more slow-drying vehicle than any of the ordinary oil varnishes, otherwise the Van Eycks could not have been able to execute their paintings with such elaborate care and finish which is so characteristic of their work. We have no evidence that the Flemish painters applied a coating of this oleo-resinous varnish to their pictures after they were finished ; on the contrary, if they had done so their paintings would not have remained in the comparatively light and fresh state as they now appear. It is much more likely that if they varnished them at all, it was with an essential-oil varnish or some " white " varnish, to which a little linseed or nut oil was added to give it elasticity some time after the work was finished.
Oil varnishes should be used very sparingly, and in the same way as the Van Eycks used them, namely, as one of the ingredients of the oil painting vehicle, or they may be used over a painted surface, if the picture is to be finished by painting again over the oil varnish, but such a varnish should not be applied as a final coating, as all oil varnishes have a strong tendency to get brown and darken in time. This darkening is not only due to the oil of their composition becoming yellower or greener, but it is also brought about by the resins or gums themselves which acquire a dark brown with age.
Oil-painted pictures should therefore be varnished with a clear white spirit or essential-oil varnish, to which a little quantity of linseed oil has been added in the making, but not until a period of from one to two' years after the work is finished, when the colours become thoroughly hard and dry, but tempera paintings may be varnished, also with a similar clear varnish, about two or three weeks after the painting is completed.
Mastic varnish is the most reliable of all kinds of spirit varnishes, but it has only a lifetime of about ten years, therefore oil paintings which are always exposed to the air should be carefully cleaned and revarnished every ten years.
The gum mastic is obtained from the mastich, or lentisk tree, Pistacia lentiscus, a native tree of Arabia, Persia, and other Eastern countries. It is a clear pale yellow kind of gum or resin, and has good drying qualities. Mastic oil varnish was in common use in the north of Germany and Flanders, from the beginning of the fourteenth century, and perhaps earlier. It was likely the white varnish " that was introduced into England in 1353, when some quantities of it were supplied by the painter Loyn, of Bruges, in that year, for the use of the painters of St. Stephen's Chapel, Westminster. This " white " varnish was provided at 9d. the lb., and the red sandarac at 4d. the lb. ; the difference in the price would lead us to infer that the white varnish supplied was mastic. In other English records white varnish is mentioned as high as 1s. the lb., while else-where white varnish is mentioned as low as 4 1/2 d. the lb. This cheaper price would lead one to suppose that the commoner white variety, made from fir resin, or concrete turpentine and linseed oil, was supplied, as these two white varnishes were not differentiated in the old records. They were generally classed under the same heading, simply as " white varnish," whether made from fir-resin or mastic, as, "vernisium alba," in contradistinction to " vernisium rubrum," the red sandarac varnish.
Mastic oil varnish was made in the fourteenth and fifteenth centuries much in the same way as sandarac, and " gloriat " (turpentine) varnishes, namely, by boiling three pounds of linseed, nut, or hempseed oils, and while the oil was boiling adding and stirring in little by little the powdered resin. Sometimes white copperas or lead preparations were added to the oil before the resins were put in, if a good drying varnish was required. In the seventeenth century and later, the Flemish painters added purified turpentine to the mastic oil varnish, and recommended a varnish of this description, not only for mixing with the colours, but also for the varnishing of the finished oil painting.
Eastlake gives an extract from an unsigned modern manuscript, which purports to be descriptive of Vandyck's method of making mastic varnish, but he says it must rest on its own merits. According to this manuscript Vandyck's receipt was as follows : " Take one pound of gum mastic, carefully picked ; powder it in an earthen vessel with two pounds of spirit of turpentine. Set this in a sand heat that is less than will make the spirit boil; let it remain (shaking it well continually) till the gum is dissolved. Take it from the fire and let it stand until the contents are cold. The varnish is to be poured out and separated from any little foulness that it may contain. Put in bottles exposed to the heat of the sun. This will make it clear and improve the colours in pro-portion to the time it is kept. Take one pound of this varnish and half-a-pint of drying oil; shake them well together ; put them in a bottle to simmer on the fire for a quarter of an hour, when the mixture will be complete. But if it should curdle as it cooks, it must be set on the fire again, and simmered until, when cooling, it does not curdle, but appears like a white jelly."
By the same writer there is a description of Vandyck's method of making a drying oil : " Take an ounce and a half, or two ounces is better, of white lead, and a pint of nut (walnut) oil ; set the oil upon the fire, in a large earthen vessel ; put in the lead by degrees, as the oil simmers very slowly over the fire until the whole is dissolved (diffused)." The oil was afterwards clarified by straining, and used fresh. Vandyck, the writer says, " never kept it by him for more than a month, for after that time it began to lose its good qualities; it is believed that Cornelius Jansen, as well as Vandyck, used this oil."
Essential-oil varnishes were used by the Italian and Flemish painters of the fifteenth and later centuries that were made from fir turpentine resins, dissolved by heat and adding an equal quantity of either petroleum spirit (naphtha) or spike oil, while the resin was in a liquefied state. This made a thin and glossy varnish, and was used as a protective covering for the colours in oil painting, to make them " bear out " without causing any of the disagreeable after darkening effects, which are always caused by the employment of an oil varnish when coated over the finished picture. Another advantage claimed for this use of an essential-oil varnish, was, that it permitted a repainting over it when dry, without any danger of cracking of the surface. Sometimes the eoncrete venice turpentine, the balsam product of the larch, was used in the making of this varnish, instead of the silver-fir resin. The latter, how-ever, is more clear and colourless than the former, and is better than the darker Venetian turpentine, unless this is used in a highly purified state.
Vandyck made and used this clear white varnish, according to the authority of De Mayerne, who was a friend of Vandyck, but in the later period of his practice, he evidently made more use of a mastic oil varnish, and more particularly so for the final varnishing of his pictures.