SECTION V
Of the fubftances ufed for rendering mater a proper vehicle for colours.
Of gum Arabic.
GUM Arabic is a tranlparent gum brought from Africa and the Levant, that is eafily fbluble in water, which it renders vifeid. From thefe qualities, and the drying with a confider-able degree of tenacity, it is the beft ingredient, for making an aqueous vehicle for colours, hi-therto known. When good, however, as it is apt to become perfedtly dry, it is fubjedt to crack and feparate from the cartoon or grounds, on which the colour is laid with it: from whence lome mixture becomes necefiary to remove this defied!.
The goodnefs of gum Arabic piuft be concluded from its tranfparent whitenefs, the being perfedtly dry and brittle, and free from all dull and heterogeneous matter. Gum Senegal, which is of lower price, is frequently fold for it: but this fraud may be eafily guarded againft, by taking care to obferve, that it be not loft and clammy * ; which qualities are always in feme degree found in gum Senegal.
There is no other preparation neceflary for gum Arabic, previoufly to being ufed with the water, except powdering it in order to its di£> fotving the more eafily: only it is proper to pick out all brown and difcoloured pieces, or any foulnefs that may accidentally be in it.
Of gum Senegal.
Gum Senegal is much of the fame nature us gum Arabic; and applicable to many of the fame ufes : but it is apt to retain feme proportion of moifture, which confequently renders at more fbft and clammy. This property makes it, when alone, a lefs fit ingredient for a vehicle zfor water colours than gum Arabic: but yet, ne- "verthelefs, a third or fourth part of the gum So? "^negal, mixed with the gum Arabic, will greatly improve it: for, as the gum Senegal is too flow -^and reluctant to the drying perfectly, fb the yim Arabic on the contrary dries in too great -«a degree; and a due mixture of them there- -^tfore produces the proper medium or tempera- ^xnent betwixt the two extremes.
The treatment of the gum Senegal, when ~wifed in a vehicle for -water colours, mufl Ise in all other points the fame as the gum Arabic.
Of fize.
Size is made of leather boiled in water till it become of a vifcid confiftgnce.
It is a very proper ingredient for a vehicle for colours ufed in water in larger works : but its reluctance to be redifiolved after it has been mixed with colours, and is become dry, renders the employing it incommodious in miniature painting. Size is commonly prepared in great quantities by thofe who make a trade of manufacturing it, together with glue, from all refufe pieces of leather and fkins of beafts: but where it is wanted for painting for nicer pur- pofes, it fhould be prepared from the cuttings and fhreds of the glovers in the following manner.
“ Take any quantity of the fhreds or cuttings t{ of glovers leather j and put to each pound ** a gallon of water: boil them fix or eight <{ hours, recruiting the water fb that it fhall ** not diminilh to lefs than two quarts. Then “ take it off the fire; and ftrain the fluid thro* ft flannel while hot: and afterwards evapo- ** rate it again, till it become of the confift- ' “ cnee of a jelly when cold.”
Of fugcr andJhgar candy.
Sugar, and white liigar candy, are ufêd ai ingredients to render water a vehicle for colours fo miniature painting. The intention of them is to prevent the colours from cracking when mixed with gum Arabic j which the fugar prevents by hindering that perfedt drynefs, and great fhrinking, which happens on the ufe of gum Arabic alone: and alio to make the gum water work more kindly with the pencil. There is no preparation of the fugar and fugar candy neceflary before they be ufed, except powdering, to make them melt the mom ealily.
Starch. .
Starch is fometimes ufed inftead of fpgajr- candy for mixing with the colours that ‘ar<^ ufed in ftrong gum water, to make them work more freely, and to prevent their cracking.: is a kind of fecret with fome perfons, whotlay aconhderahleftrefs upon it. • '
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, Of i/inglafs. ■
Ifinglafs is a.glue formed from the cartilages of a large fifh, which is found in the rivers diat How into the northern feas. It is ufed in mi4 mature painting, in the fame manner as the gums Arabic of Senegal, for rendering water a proper vehicle, by giving it a due vifcidity for fpreading and binding the pigments of an earthy texture. ,