NAPLES-YELLOW, called also Neapolitan earth, in Italian Giallolino, and in French Faune de Naples, is a beautiful pigment, concerning which we have much information from the indefatigable Beckmann. "It has (says he) the appearance of an earth, is of a pale orange-yellow colour, ponderous, granulated, exceedingly friable, does not effloresce, nor become moist when exposed to the air, but when applied to the tongue seems to adhere to it. When reduced to a fine powder, it remains for some time suspended in water, but soon deposits itself at the bottom in the form of a slime. When boiled with water, the water, at least sometimes, is observed to have a somewhat saline taste. It does not effervesce with acids, but is in part dissolved by aqua regia (nitro-muriatic acid). In the fire it emits no sulphureous vapour, is difficult to be fused, and by that operation undergoes no material change, only that its colour becomes somewhat redder. When fused with colourless glass, it gives it a milk-white colour, a sure proof that it contains no iron; and, with inflammable substances, there is obtained from it a regulus which has the appearance of a mixture of lead and antimony."
"This article is brought from Naples for the most part in the form of an earthy crust about three or four lines in thickness, and it sometimes retains the form of the vessel in which it has hardened. It can be procured also as a fine powder, as the colourmen keep it sometimes ready pounded for use."
About the nature of the substance called Naples yellow there has been much diversity of opinion. Most of those who have written about it, consider it as originating from fire, and as a volcanic production of Mount Vesuvius or Mount Ætna; others have pronounced it to be a natural ochre. Guettard thought it rather a kind of bole; but Pott approached nearest the truth, by asserting it to be an artificial preparation*. Pon. * Lidge- geroux is entitled to the merit of having proved this, 22mo, v. 2. and of having shewn the possibility of preparing it. Ac. P. 15. According to his experiments, Naples yellow will be obtained, if you boil for seven or eight hours, first over a slow and then over a strong fire, a mixture finely pulverised of twelve parts of pure white lead, one part of alum, one part of sal ammoniac, and three parts of diaphoretic antimony† (white oxyd of antimony by nitre). But before Fougeroux, who may have obtained an account of the process during his travels through Italy, a more certain process was published in the year 1758, by Giambattista Passeri, in his interesting work on the painting of earthen-ware‡. The articles to be employed, according to this author, are, "one pound of antimony, a pound and a half of lead, one ounce of alume di feccia, and the same quantity of common salt." I am inclined (says M. Beckmann) to think that this receipt was not unknown to Fougeroux, and that he considered alume di feccia to be alum. Professor Leonhardi, a man of very sound learning, has translated this expression by the word alum. I will, however, freely confess, that I consider alume di feccia not to mean alum, but salt of tartar, or potash. Passeri
feri says, that the proportions may be varied different ways; and he gives six other receipts, in which he does not mention alume di feccia, but only feccia; and this word certainly means weinbesen or winestone (tartar). Professor Leonhardt himself seems to confirm this opinion, by saying, that Vairo, professor of chemistry at Naples, has translated "the ashes of wine lees" (cineres inferiorii) by the words alume di feccia.
After Fougeroux's paper was printed, De la Lande published a receipt which he had received from the well-known prince San Severo, and in which lead and antimony only are employed; but no mention is made either of alum, tartar, or any other salt. This receipt is as follows:
Take lead well calcined and sifted, with a third part of its weight of antimony pounded and sifted also. Mix these substances well together, and sift them again through a piece of silk. Then take large flat earthen dishes, not varnished, cover them with white paper, and spread out the powder upon them to the depth of about two inches. Place these dishes in a potter's furnace, but only at the top, that they may not be exposed to too violent a heat. The reverberation of the flame will be sufficient. The dishes may be taken out at the same time as the earthen-ware, and the substance will then be found hard, and of a yellow colour. It is then pounded on a piece of marble with water, and afterwards dried for use.
The enamel-painters in Germany prepare a yellow glazing, not very different from the real Naples yellow, by a prescription, according to which, "one pound of antimony, six ounces of red lead, and two ounces of white sand, are to be fused together. The produce, which appears quite black, is to be pounded, and then fused again; and this process is to be repeated till the whole mass becomes thoroughly yellow. Half a pound of this mass is to be mixed with two ounces of red lead, and afterwards fused; and by this tedious process an orange-yellow pigment will be obtained."
All artists who speak of the use of Naples yellow, give cautions against applying iron to it, as the colour by these means becomes greenish, or at least dirty. For this reason, it must be pounded on a stone, and scraped together with an ivory spatula. It is employed chiefly in oil painting, because the colour is softer, brighter, and richer than that of ochre, yellow lead, or orpiment, and because it far exceeds these pigments in durability. It is employed in particular when the yellow ought to have the appearance of gold, and in this respect it may be prepared with gum water, and used as a water colour. A still greater advantage of it is, that it is proper for enamel painting, and on that account may be employed on porcelain or earthen ware (A). Professor Beckmann, however, recommends to artists to examine whether the oxyd prepared from wolfram, by boiling in the muriatic acid, which has a beautiful yellow colour, might not be used in the same manner as Naples yellow.