Violin Varnish and How to Make It/Chapter 5
CHAPTER V.
Dry Substances forming the basis of Varnish.
GUMS are as a general rule easily dissolved in water.
Gum-Resin, which is a mixture of gums and resins with the addition of volatile oils, salts and water is insoluble in water. Gums are derived from vegetables of numerous kinds.
The product of each plant enables us to distinguish the variety. Rosins are also obtained from vegetables, and in such quantities that the manufacture has developed into a large and increasing industry.
There are two methods of extracting rosin from plants.
One way is simply by collecting the natural secretion, and the other by provoking such secretion by making incisions in the tree at certain periods of the year.
Rosins when pure are usually without taste or odour—white, yellow, or brown in colour. They are insoluble in water but can be dissolved to a certain extent in alcohol, ether and various oils. Oxygen has not much effect on them with the exception of the copals.
It is very difficult to obtain a pure rosin, it is nearly always a mixture of different rosins, each having separate properties.
One great difficulty to be met with in the making of varnish is that rosins are not always of the same degree of hardness.
This applies to such an extent that various rosins are classified under two heads, namely, hard and tender.
Under the heading of hard we have copal, shellac and amber. The name tender is given to such rosins as sandarach, mastic and dammar.
But there are some copals which are neither hard nor tender, but are what might be termed semi-hard. It is difficult in spite of these descriptions to obtain in the trade an article that will correspond accurately to the various requirements that may be needed.
Gum Arabic.—There are numerous varieties of this gum, the distinctive character of which is its solubility in water and non-solubility in alcohol. It is to be found in Africa, India and Australia.
It is extracted from the different species of Acacia, but many fruit trees produce similar gums.
Rosins—Copals represent a great number of rosins which vary very much in constitution. Anime is but a variety of rosin, and dammar is also but a series of similar products. Copal is much employed in the manufacture of all kinds of varnish. It is derived from certain vegetables grown principally in Africa, and as they are sent to us mixed indiscriminately it is with great difficulty that the manufacturer can distinguish between them, consequently they can seldom repeat a varnish identically from two deliveries of rosins.
It is also obtained in small quantities from India, America and Australia.
It is a hard substance which, after being subjected to the process of melting, becomes partially soluble in spirit and to a higher degree in ether and essence of turpentine.
Correctly speaking it is not a pure rosin, but a mixture of rosins each having separate degrees of solubility.
It also contains some oils.
| Hard copals | will | melt | at | 350° | centigrade. |
| Semi-hard | will„ | melt„ | at„ | 150° | centigrade.„ |
| Tender | will„ | melt}„ | at„ | 100° | centigrade.„ |
Hard copals mostly come from Zanzibar. Those known as animes reach us from Bombay and also Madagascar. The semi-hard are obtained from Africa and reach us in different forms. Those coming from Angora are in the form of reddish balls.
Those from Benguela (Africa) are of two different kinds, one yellow in colour and of a flat shape similar to a shell, and the other of a grey colour and is sometimes called African gum. Copal from Sierra Leone is white and elastic and is especially adapted to be mixed with other varieties in the manufacture of varnish. Asia supplies us with many copals but these are not genuine Asiatic products, but are sent from Africa, mixed in Asia and from thence exported to Europe.
The real copals of Asia are divided, as are the others, into hard and tender. The best African copals are the hardest, they much resembling amber. They are easily distinguished from it, however, as they will readily melt if brought into contact with the flame of a candle.
There are many kinds, the following being the best known:—Zanzibar, Madagascar, Cape of Good Hope, Sierra Leone, Congo, and Angora. The solubility of copals has been mentioned before when describing the rosins. They are a mixture of various matters of similar constitution, but which differ in several essential points, notably by the variations in the degree of solubility.
Certain other bodies mixed with the rosin will greatly facilitate their dissolution in alcohol, camphor and ammonia being generally used for this purpose. The process is very gradual, but camphor is extensively employed by most makers.
Anime.—This copal is the produce of America, Brazil and Cayenne. It is very hard, but readily dissolves in alcohol.
Dammar.—This comes from Australia and New Zealand, is partly soluble in alcohol but not completely so by the cold process. It can, however, be entirely dissolved in boiling alcohol and essential oils. It is one of the most tender of copals. It is also found in Batavia, and this kind is favoured by many on account of the whiteness of its colour.
Gum Benzoin.—This is a compound of many kinds of rosins, some acids, and a certain amount of essential oils. It is easily dissolved in alcohol. The best qualities are obtained from Siam.
Shellac.—This is a rosin which differs in every respect from any other kind. It is somewhat similar to the cochineal, it being the produce of an insect that lives on certain plants. It is soluble in alcohol only, to which it gives its colour. From it, is made the French polish which is mostly used in the furniture trade.
Mastic.—There are two sorts of mastic, that in tears being the purest and best. It comes from Africa, and is partly soluble in alcohol and completely so in ether and essence of turpentine. Mastic is used in all varnishes to render them pliable and to make them dry slowly.
Sandarach.—This is a product of Algeria and Morocco. It is quite soluble in alcohol and essence of turpentine and is extensively employed in the composition of spirit varnishes.
Dragon's Blood is procured from India, Ceylon and America. It is soluble in alcohol, ether and oils. It imparts to its solvent a rich blood red colour.
Amber.—This is a resinous fossil and is found in many countries. It is generally understood to be the petrified remains of certain trees that are now extinct.
It is hard, yellow in colour and transparent.
Its composition is complex consisting of three different kinds of rosins, some essential oils and a small quantity of mineral matter.
In its natural state it is not soluble in alcohol nor essence of turpentine nor essential oils.
It can, however, be dissolved by the following process.
Place some in a clean earthenware vessel and heat it gradually, when melted pour on to a marble slab, when it becomes hard again reduce it to powder.
It can then be readily dissolved in spirit, essence of turpentine and also in all the oils extracted by the distillation of tar.
This method is very old and can be applied to many hard rosins not soluble in their natural state.
Great care must be taken not to apply the heat too long or too fiercely, otherwise it will burn and so spoil the colour and deprive it of its properties.
Amber is much used in the manufacture of the most durable varnishes, particularly that used by painters and decorators.
Turpentine.—This is an aleo rosin and is the produce of certain plants of the coniferae family. It contains varying proportions of essential oils.
The commonest type is the most siccative, and by the addition of one part to sixteen of magnesia it will become hard.
It is completely soluble in alcohol. Another variety, Venice turpentine, is not siccative but easily dissolved in spirit.
It is a mixture of essential oils and rosins.
Elemi.—Elemi may be classed as a rosin. It is obtained from Brazil and Mexico. It is partially soluble in alcohol, water or ether.