USED IN PAINTING.
SECTION III.
Of the general nature and application of . thefubflances ufed in enamel painting; with their previous preparation.
Of the Jubflances if ehfor forming fluxes.
MINIUM or red lead is ufed, as a flux-ing body, for forming the enamel for grounds; as alfo in compounding fluxes for the colours. It requires no preparation for thefe
purpofes; only it is proper, it fhould be pure; which may be known by the method before given p. 48. This flux renders the enamel foft} but producing feme proportion of yellow colour
is not fit for all ufes.
Fixt alkaline fait of vegetables is fometimeS ufed alfo in forming the mixture for enamel grounds; as likewife in fome compofitions of fluxes for the colours. It makes a lefs foft enamel than the lead; but is free from yellow, or any other colour; and therefore proper for fome purpofes.
• Borax is a fait of very peculiar qualities; a- mongft which, is that of promoting vitrification, and the fufion of any glafs when vitrified, in a greater degree than any other fubftance known ; on which account it is of the greateft confequence, in forming fluxes for enamel. It requires neverthelefs either to be previoufly calcined or brought to a vitreous Rate, which it fuffers from the application of moderate heat a-lone ; and then finely powdered before it be mixed with other ingredients in fluxes. Its ofe is -not much known in common prattioe; though of the greatefl confequence to the art o? enameling; as not only a fet of fbfter colours may be produced by die aid of it, ,than can be otherwife had ; but the degree of each may be brought to correfpond, by the employing it in different proportions according to the refpeftivc hardnefs of the other ingredients, which differs lb much, as not to be regulated juffly by any other means.
Common fait may be alio ufed os a Aux in enameling, particularly where there is oOeofioa for glarings : where, as k is extremely fluid, and free of tenacity when fufed, as alfb lefs fub- Jc&to crack than any other vitreous body, k is of great ufe. But for fluxes for grounds and colours in enamel it is not frequently necefl&ry to multiply ingredients, as the above three futn fiances may, when properly applied, fufficient- ly anfwcr moil purpofes: and the fame reafort- ing extends to nitre and arfenic j which though they have the qualities ef fluxes, poffefsyeta- long with them fuch other, with refpeft to .their effedt on feveral of die fubflances that com- pofe the colours, as renders the methods of vtfing them difficult and complex. ,
©ƒ the jhijhnces ufed for forming the indy <f enamel, or fluxes.
i . -
White fend is ufed as a body for the fluxes ted,grounds of enamel; it fhould be reduced pre-vioufly to an impalpable powder, in order that k may be mixed more intimately with the other ingredients ; which not only accelerates the vitrification; but renders the glafs greatly ntore perfect. The kind of fend proper for this pbfpofeis that brought from Lynn in Norfolk i sad called by the name of that place.
Flints are ufed for the fame purpofe as the white fend : and it is proper to ufe them, when that cannot be procured of the right kind; They require to be calcined before they are applied to any pttrpofe of Vitrification: which is to he dofie, by putting them into any fire, and continuing them there till the whole fob* ftante become white : when they muft be taken out; and, while of their full heat, im- merfod in cold water; and kept there for fome time ;: by which treatment, they will be rendered ■of A very brittle and caicarious texture and wery eafy to be powdered: which muft be done to a perfect degree for the reafon above given. Where fhvall quantities of the matter of any kind of enamel is to be prepared, calcined Hints Are preferable to fend; as they are much more eafily reduced to an impalpable powder, and the trouble of the .previous calcination is very Utfile>
There is a fort ef ftone which the French call motion, that forms the upper cruft, and lies round the free ftone in moft quarries. This ftotje will lofe its tenacity in a moderate fire, and when calcined, runs much fooner intovi- trifecanon thun either Hints or fend. It is there-fore, when it can be obtained, a better matter for the body of fluxes or (oft enantel than either of the other: as it will, with the lame proportion of the fluxing ingredients, make a much fofter flux; or it otherwife, admits of the diminution of the proportion of lome of them j which, for reafons we fhall fee below, is in certain cafes an advantage.
Of the fubjlances ufedfor producing a white colour in enamel y or for forming the grounds.
Putty or calcined tin is ufed as a body of lour for the enamel grounds. As tin is very troublefome in calcining, requiring a long continuance of fire, and to be fpread into a very large furface, it is much the beft way to procure it for the purpofes of enameling ready calcined, of thofe who make it their proper bufinefs to calcine it, for the ufe of lapidaries, and other artifts who ufe it ; as they have large furnaces, fitly conftrudted for per* forming that operation in large quantities, and can confequently afford it much cheaper than it can be prepared in fmall quantities ; be- fides the fparing the trouble. It mull be demanded of them by the name of putty 3 and care mull be taken that it be not fophifHcated: •which it feldom fails to be before it comes out of their hands for common purpofes. The fb- phiftication, which is generally by chalk, lime, or feme fuch white earth, may be dilfinguifhed by putting the putty into a crucible with fome i tallow or other greafe ; and giving it the heat of fufion, or what is fufficient to melt it ; fupply- ing the greafe in frefti quantities as it burns away, till the calcined tin appear to have regained its metallic (late : when fuffering the remainder of the greafe to burn away, the chalk or earth, if any were mixt with it, will be found fwiming on the furface of the metal : to which however the allies of the greafe mufl be fuppofed to have added feme little quantity. -There is, neverthelefs, another body with which, the putty or calx of tin may be adulterated, that will not difcover itfelf by , this .method of reduction of the tin. It is white dead, which, in this manner of treatment, would run into fufion, and mix with the tin j and could therefore not be diftinguifhed from it. But it may be eafily rendered perceptible by another manner of proceeding : which, is to take the putty fufpe<fted to be adulterated with it, and having put it into a crucible, without any admixture, and inverted another crucible over it as a cover, to give it a moderate heat, carefully avoiding that the fmoke or coal of the fire may have any accefs to it to change its colour. If there be any white lead mixed with the putty, it will Ihew itfelf, when removed from the fire and become cold, in a yellow or brown colour: and if no fuch colour fupervene, but die putty appear equally white as before it was heated, a conclusion may be fafely made, that it was not adulterated by , \vh|te lead ; .but, . if fophifticated at all, by feme white earth, which may be made ptr* ceptible by the reduction of tin in the man* ner before mentioned.
Where a very pure white is wanted for on* amd, the eafieft and heft method is to oakam the tin by means of nitre} which may be thus done.
“ Take half a pound of fait pttre, and ** put it into a meldng pot, fuch as is de* “ foribed p. 240; and put it into a foe to “ mdt it. When it is melted, throw in gra- rt dually ten ounces, ^r a half of a pound of ** tin filings, which fhould be rafped as fine “ as poifible $ but give time for the expiofhm, “ dint will follow, to ceafe betwixt bach “ quantity that is thrown in ; ftiiring, how- “ ever, the matter in the mean time with the ** end of a tobacco pope. After the whole is “ put into the melting pot, for it again well “ about for feme time; and then take it tart ^ of the fire; pouring all the matter out of ** the pot, that can be got from it by that “ means j and then {bsk the pot in water till “ the remainder be foft enough to be foraped “ from it: taking great care not to let any “ part of foe fobfomce of the )pot be txato- “ mixed with foe calcined matter. That “ which is maidened in Order to be got oat “ of foe pot, muft be dried, and put to foe “ other, and the whole wdl pounded; amd “ kept flopt up in a bottle for ufe. There ** is no occafion for edukoratiop, or wa tiling
the £dt$ from foe calx, far this purpofc, 2 ,becaufe they are by no means detrimental,“ but rather advantageous to the enamel.” Antimony has been alfb applied to the lame ufe as tin : but the expence and trouble of reducing it to a calx, which mud; be by deflagrating it with nitre, renders the ufe of tin much more expedient. Merret in his notes on Neri, recommends equal parts of die antimony and nitre; but as that proportion does Hot calcine the antimony to whitenefs, but produces the crocus metallorum, or liver of antimony, which is of a foul orange or reddilh yellow colour, it is by no means fufficicnt: he fays likewife, that regulus of antimony will aftfwdr die feme end; but in this he is dill further midaken *, for the regulus, winch is a metallic body, in feme degree malleable, could never be reduced to powder, as he directs all the ingredients in the competition he prescribes 'to be : nor if it were, would it form a white body on the fofion with the other matter.
When antimony is ufed for the colouring white enamel, it fticrald be previcmily calcined by means of nitre, in the following manner.
“ Take of antimony one part, felt petre “ three parts.* Powder them well together j “ and then throw it, by a fpoonful at a time, ** into a crucihle heated red hot j waiting be“ twixt each time till the explofion the mix“ ture will make be entirely over. When ** the whole matter, is put into the crucible, “ and' has remained feme time in a quiet “ date, take it Oit of the nre j and proceed
*.* in all relpedts, as was above directed for the “ tin, when calcined in this manner.”
The calx of antimony fo produced will be of a finer white than the calx of tin can be eafily brought to, unlefs by this method of calcination: and therefore more fit for pur- pofes, where great purity of colour is wanted, either in grounds or painting. But as the tin prepared in the fame manner may be rendered very white, it will be lefs expenfive, as it requires lefs nitre to calcine it, and produces a much greater proportion of calx than the antimony.,
Arfenic is alfo ufed for forming a white colour in enamel: but it is a very nice matter to manage it well; as it is very loon changed by the heat into a tranlparent body, being itfelf a ftrong flux; and it is therefore much better to omit the ufe of it, unlefs for feme particular purpofes, in the Hate of white glafs, as I lhall below have occafion to mention. 'Arfenic is allb ufed as flux: but its effedts on lome colours renders it not very fafe without great knowledge of its qualities, and caution in its application.
Of the fubjlances itfed for producing red, yellow, &c. colours in , 1 Ultramarine (the preparation of which we have before given) is ufed in enamel, where .very bright blues of a- lighter teint is wanted j and fometimes, indeed, in other cafes, by * thofe who do not underftand die right ufe of zaffer and fmalt: but there are few inftances where zaffer, when perfectly good, fluxed with borax and a little calcined flint, or Venetian glafs, to take off the foluble quality of the borax, will not equally well anfwer with the beft ultramarine. The ultramarine requires no preparation when ufed in enamel painting,* previoufly to its being mixt with the proper flux: and what relates to its general qualities, and the means of diftinguifhing its goodnefs or genuinenefs, we have, along with its preparation, before taught, p. 67, and the following.
Zaffer is ufed for producing blue, green, purple, and black colours in enamel. Jt i9 an earth obtained by calcining a kind of ftone called cobalt; and when it is mixed with any kind of vitreous bodies, it vitrifies; and at the fame timeaffumes a ftrongblue colour j but for the moft part verging to the purple. It is to be had, in a ftate proper for ufe, of thofe colour- men who make it their particular bufinefs to fupply the glafs-makers with colours. The goodnefs of zaffer can fcarcely be known but by an adtual trial of it; and comparing the effedl of it with that of fome other known to be good and ufed in the fame proportion.
Magnefia is an earth, which, when fluxed with any vitreous body, produces a broken crim- fon, or foul rofe colour. It is to be had, prepared fit for ufe, except a more perfedt levigaticn from thofe who fell colours to the glafs-maker. It is ufeful not only for fome purpofes . as a red, but for feveral compofitions for black, purple, and forne browns. The good- nefs of magnefia mull be determined, by the feme means as that of zaffer.
Smalt k zaffer vitrified, with proper additions} which are generally fixt alkaline felts and fend, or calcined Hints; and is fometimes ufed as a blue in enamel : but being hard, it requires, for fueh purpofes, to be ufed with a flux; which increafing the body of glafi in too great a proportion for the tinge, is apt to dilute the colour too much, where great force is wanted: and therefore the ufe of the zaffer itfelf is in moil cafes preferable. Smalt, however, ground very fine, and mixt with a fourth part of its weight of borax, (which is much the moft powerful and kindly flux for zaffer) will run pretty well, and may he ufed where either a full colour is not demanded, or where the work will admk of the colours being laid on thick. The gcodnefe of fmalt may be judged of by its bright and deep colour; and the Ids k inclines to the purple the better. In order to judge of die ftrength of the colour, the fmalt fbould be reduced to a fine powder: for in a grofler Hate, eveiy degree of finends renders it fo different, that a judgment cannot be eafily formed of k. Smalt is to be had of all colour men, and is not fob»» je<ff to adulterations, which would not be obi. vious on inlpe&ion.
Gold is ufed in enamel to produce a erim- fon or ruby cofoer.; whjch, by the miftaken fenfe of die Latin ward furptratty has been catted pur}]# by alt the EnglUh and French writer^. It muftbe previonfly reduced to the Hate of a precipitated powder, by dificdving in aqua regia; and making a precipitation by means of tin, fixt alkaline iak, or fome othes metallic or alkaline body. There has been feveral methods «»led fee the making this pro* eipitate of gold; but the following will pen* fe&ly anfwer the end with great cafe and cen* tamty.
■** Take of pure fpiiit of nitre eight ounce». u Add to k of fal ammqn henna fcraped perfect* M ly clean and powdered two ounces; which u will convert die fpiritof nitre to aqua regia) “ Diflbtve, in fear ounces of this-aqua regia, H put into a proper phial, half an ounce of “ purified gold, in the date, k is to be had of ** the refiners, under the name of grain gold. *' In order to haften this, fetation, the phial " may bo put into a gentle heat, where it
mu« continue till the gold iatirely diiap* M pear. Take, in the mean time, about the H lame quantity of aqua regia, in another phial j w and put into it filings or finaU hits of pure “ block tin, fo long as any briflc. effervefcence ** arifes on the adding frefh quantities: but w this mud be done gradually, efpecially if *i* the filings, be ufed; otherwife the mixture *5 wiH heat fe much as to boil over or break * die phial. Drop then thirty or forty drops of u die fetation of the gold into a half pint glafc " of water; and immediately after about fifteen or twenty drops of the folution of tin. The “ gold will be men precipitated in a red pow- “ der froth the iolution in the aqua regia ** dropped into the water ; and this operation “ muft be repeated till the whole quantity of **• the folution be thus treated. When the laft quantity of the red powder has been preci- “ pitated, pour off the clear fluid; and fill “ the glafs with fpring water ; which, when «* the red powder has fettled, muft be poured “ off likewife. Hold then a fponge wet, but ** well fqueezed, to the fùrface of the fluid re“ maining with the powder; and when as “ much of the water as can be conveniently ** feparated from it, by that means, is drawn “ off, lay the powder on a marble or porphyry “ ftone to dry : taking great care that it con- “ tra£t no duft or foulneis.” -
Inftead of ufing the folution of tin to precipitate the gold, the crude tin is moft frequent* ly employed : but the precarioufiiefs of this method, which requires much more attention* overbalances the troublé of making the folution. For if the folution be mixed with as much water in this method as in the other, it is very (lowly aéted upon by the tin : and if the folution be not diluted with fo much water, it forms a gelatinous body, when ftrongly (aturated with the tin, which can ' never be feparated from the precipitated gold, but by means that are deftruétive of its qualities as an enamel colour. When the crude tin is ufed, however, the folution muft be diluted with about treble its quantity of water $ and the tin rauft not be longer continued in it, than while the gold appears to form a red powder on the furface of it, on its being freed from that which before adhered to it. It is better, neverthelefs, on the whole, to ufe the two folutions: as it. is more eafy to preferve a fcarlet colour, by that means: for if the tin be too long contihued in the mixture, it gives the colour a tendency to the purple. When a red colour is wanted, which verges greatly on the purple, a precipitation of the gold fhould be made by means of any fixt alkaline fait. Which may be thus done.
“ Take the folution of gold in aqua regia, “ as before directed: and drop in it a folu- “ tion of fait of tartar (which muft be made “ by melting half an ounce of fait of tartar in “ a quarter of a pint of water) fo long as “ there appear any effervelcence or'ebullition “ or the further addition. Let the precipi- “ tated powder then fettle; and proceed as “ was above directed for the calx Caflii, or precipitation with tin. The powder thus “ produced is called aurum , from
its quality of exploding when expofed to a “ moderate heat: which muft therefore be ** carefully guarded againft in the ufe of it,
“ by keeping it out of the reach of any fuch ** heat till it be mixed with the flux for enameling: and it will be the lefs hazardous “ from being thoroughly well freed from the fait, formed in its production, by walking.
The gold maybe like wife precipitated, in the feme manner, by volatile felts: in‘which cafe the volatile felt in the proportion of half the weight of the aqua regia may be diflolved in four times its own weight of water. But this method does not produce lo fcarlet a red in the gold precipitation, as that of ufing the folution of tin, as before directed.
This precipitation may be allb made by mercury diflolved in aqua regia; and it is laid that a finer colour is produced by this method than with tin: as likewife, that if the aurum fulminans, or any of the other precipitations, be fufed with common fulphur, they will be rendered of a much brighter red; but the fulphur muft in this cafe be fuffered to burn away. Thefe methods, neverthelefs, are attended with much more difficulty and hazard than the fimple method firft given; and, perhaps, unlefs by accidents not be commanded, will not produce a better pigment for enamel painting.
Befides the application of gold to form a red colour, it is uled to produce the eflfeft of gilding in enamel j for which purpole it muft be reduced to the ftate of a precipitated powder in the following manner.
“ Take any quantity of gold dillblved in ** aqua regia, as mentioned above. Put ** into it long flips of copper plates; and con- “ tinue them there till the gold no longer “ appear to form itielf in a powder on their
“ furfece:
** furface : in order to the obferving which, “ the gold ' already cohering, mull be from cc time to time fhaken off -, or they may re“ main till they no longer appear to .excite “ any eflfervefcence or bubbling in the fluid. “ The flips of copper being then taken out, “ the water mull be poured oflffrom the pre- “ cipitated powder, and lèverai frefh quanti- “ ties added to free it intirely from me fait “ formed by the aqua regia and copper : after “ which it may be dried, and will then be “ fit for ufe.”
Silver is ufed for producing a yellow colour in enamel. It muft be previoufly reduced to the ftate of a powder : which may be done either by precipitation from fpirit of nitre, or by calcination with fulphur. The precipitation of filver from fpirit of nitre, may be performed by diflolving an ounce of filver, in two or three ounces of fpirit of nitre ; and precipitating and edulcorating it exactly in the fame planner, as was above.directed for precipitating the gold from aqua regia, by means of copper for gilding in enamel. A precipitation may otherwife be made by pouring brine on die folution of filver in the fpirit of nitre : but I think the other method preferable. The calcination of filver with fulphur may be thus performed.
(t Put plates of filver into a crucible, with “ as much of the flowers of fulphur betwixt “ them as will cover the furface of each plate : “ and then place the crucible in a fire, that
•* will heat it red hot. When being taken
“ out, the filver will be friable or brittle, and
“ muft be reduced to a fine powder in
“ mortar of glafs, agate, or flint. The cal-
“ cination may be otherwife made, by mix-
“ ing filings of filver with flowers of fulphur,
“ in the proportion of one ounce of the filver
“ to half an ounce of the fulphur; and heat-
** ing them red hot in a crucible; or the ful-
“ phur may be thrown in to the crucible after
the filver is already made red hot.”
Copper is ufed, in enamel painting, for the forming green, blue, and red colours: but it mull be previoufly either calcined, or reduced to the flate of a powder by precipitation.
The calcination may be performed, by means of fulphur, in the fame manner as is above directed for filver: but it requires a ftrong fire for two hours: when the copper will be found converted into a blackifh red powder, which mull be well levigated by grinding and lifting.
When the copper is fo prepared, it is called Ferretto of Spain. It may be otherwife cal-cined by flratifying it in the fame manner with Roman vitriol: but a much longer continu-ance of fire is required in this method : and Neri fays it ought to be fix times repeated to have the ferretto perfectly fine. When all this is done, neverthelefs, the fubftance pro-duced cannot be really different, notwith-ftanding the intimation of Neri to the contra-ly, from that of copper calcined with fulphur if the quantity of fulphur employed be fmall,
" And die time of the calcination well adj lifted1; which muft be judged of by the goodnefs of the ferretto when prepared; die criterion or mark of which, is its appearing red when levigated: for if it verge to the black or purple, either the calcination has been too long continued, or the proportion of fulphur employed was too great.
. Inftead of crude copper, the kind of latten called, by artificers in metals, ajfidue^ may be «fed: but as the plates of it are too thin to be treated in the way of ftratification, or layers, as the quantity of fulphur ufed would be much too great a proportion, it is better to clip the aflidue with feiflars into fmall fhreds; and mix it by that means with the flowers of fulphur : and the extreme thinnefs of the plates, in this cafe, renders the neceflary time of calcination very fhort.
Copper or brafs in thin plates, latten, or aflidue, may be, likewife, calcined without fulphur, by expofing them to a ftrong heat . for a confiderable time. But as foon as the heat has rendered them friable or brittle enough to bear levigation, it is the belt way to powder the matter, and place it again in the fire thinly fpread on a tile, or other fuch convenient thing; flirting it fometimes, that every part may be expofed to the open heat; and, by this means, the calcination may be much accelerated: a due regard fhould be had likewife in this cafe to the red colour of the calcined matter, as well as in the cafe of that calcined with fulphur.
But, notwithftanding, that it is requifite, for many purpofes, to nave the copper calcined only to a ftate of rednefs: yet it may be expedient, likewife, for fome particular ufes, to prepare other quantities with a higher calcination $ which muft be continued till the copper appear a dark purplifli grey, or light black, when powdered: but it muft yet retain fome tinge of the red: for if the calcination be puftied beyond that point, the calx becomes very difficult to be fluxed; and does not afford any colour in a kindly manner to the enamel.
The other method of reducing copper to an impalpable powder, is by precipitation : to which end, the copper muft be diflolved in any acid, (for all will diflolve it) and precipitated, by adding of a folution of pearl-affies in water, in the lame manner as was direfted p. 86, for making the kind of verditer called Sanders blue, except in the ufe of ftarch, which muft be here omitted: and for making green colours in enamel, this will be found preferable to the calcined copper.
To avoid the trouble of difiolving the copper as above advifed, Roman vitriol, which is only a combination of copper with oil of vitriol, may be ufed in the place of fuch folution. It muft be previoufly diflolved, by adding hot water to it in a powdered ftate : and then the copper may be precipitated, by
means of pearl-afhes, in the fame manner as from any other folution of it.
. Iron is ufed to produce an orange red, or foul fcarlet colour in enamel; as alio a tran- fparent yellow} and to affift, likewife, in the formation of greens, and other compound colours. It is prepared many ways, both by corrofion, and precipitation ; fome of which indeed make a real difference, but moft of them lead to the fame end. The only difference in faft, is, that when, the iron is highly calcined, and freed in a great degree, not only from all acid, but even its own fulphur, the appearance of the crude calx will verge moft upon a purple colour j and produce a foul purple enamel, if compounded only with a quantity of flux not fufficient to vitrify it; but when compounded with a greater quantity«- of flux, will vitrify into a tranfparent yellow, ibmewhat inclining to the red: whereas when it is lefs, or not at all calcined, but retains its own fulphur, Or perhaps fome proportion of acid ufed in the preparation, it will in proportion be yeilow, or verge towards the yellow, when ufed with thé left proportion of flux, and produce a cooler or left red yellow, when ufed with a quantity fufficient to vitrify it. Inftead of ufing the crude iron in thefe preparations, where it is to be precipitated, or calcined, it is much better to ufe common green vitriol; which confifts only of iron and me acid of vitriol; from whence con- fequently the iron may be obtained in the ftate, to which thefe preparations lead, by eafier and lefs expenfive means, than when ufed crude. But the preparation of the ruft, formed by vinegar, requires the iron itfelf; and if found necefiary, is, neverthelefs, the only inftance where the precipitated bafis of vitriol will not anfwer the fame end as the iron.
The firft preparation of iron is, therefore; the ruft by corrolion with vinegar, which may be thus made.
“ Take of iron filings any quantity; the “ finer they are the lefs trouble mey will give “ in the preparation; and fprinkle them with** vinegar, rubbing them together after it is “ added, that every part may be moiftened “ equally. Spread them, in any cool place, “ where they may be free from duft, on a V* board or paper; and let them remain there ** till the moifture appear to be dried away: “ and then try if they be fo corroded as to bear powdering: which, if it is found prac- “ ticable, mull be performed on a porphyry **. ftone with a muller, or in a glafs or agate “ mortar: but if they appear not fufficiently ** corroded, they mull be again moiftened with vinegar; and laid out as before; and, ** when become fit, powdered in this manner. “ The powdered ruft muft then be fitted through a fine fearce; and die grofler part, *c which will not pafs, moiftened again with ** the vinegar: which muft be repeated till 11 none of the iron worth further notice neu main uncorroded. The whole muft then “ again be levigated, till it be a perfectly im« palpable powder; which will be then fit “forufe.”
The iron prepared thusby vinegar, is proper for making a tranfparent or glazing yellow in enamel: or for compounding with blues to form green colours. But this procefs is greatly more troublefome and laborious than thofe below given: and is attended with no benefit in the produce, except , that this ruft will afford a cooler or lefs red yellow than the others; and may therefore be of advantage in fbme particular cafes, for forming very bright greens.
It has been ufual to calcine the ruft, prepared thus with Vinegar, to form what is called the crocus martis; but it is a very injudicious method of proceeding ; becaufe where calcination is to be ufed, the vitriol, or the iron corroded by fulphur, are equally good, and fave a confiderable trouble and delay.
Iron is fometimes calcined per fe, that is, without any mixture, by expofing the filings fpread with a large furface to the adtion of flame for a confiderable time; which converts the irorvinto a crocus martis, that, when levigated, is fit for ufe. But this preparation is alfb troublefome, and inconvenient; requiring a ftrong and continued fire: and, when made, affords nothing but what may be much eafier obtained by the methods below given.
Iron is alfb calcined by means of fulphur, which rauft be performed in the fame manner as was directed above, p. 266, for the cai' cined copper. There is not, neuerthelefe, any difference betwixt this and the calcined vitriol.
The precipitation and calcination of green vitriol are the mod; expedient preparations of iron ; and anfwer all the purpofes of the others folly, except in the inftance before mentioned of not producing quite fo cool a yellow, as the ruft formed by vinegar. The precipitation of vitriol may be formed in the following manner. :
“ Take any quantity of green vitriol; and “ diflolve it in water. Add toit gradually a ** folution of pearl-afhes in water, (which »* need not in tins cafe be purified if the felts cf be clean) till no more effervefcence arife “ and then pour off the fluid when the pre** cipitated powder has fettled.. ■ The remain- “ mg fluid, winch cannot be poured off, may “ be feparated from the ponder by means ** of a filter; and the powder then dried: *.* for as the felts will be no way injurious to “ the enamel, there is no occafion for wafh-
ing in this cafe.”
This oker or precipitated iron will nearly anfwer the feme end as the ruft by vinegar; and will afford a tranfparent yellow almoft as cool: this is therefore the beft and much the eafieft preparation of iron for forming greens by the admixture of blue.
The calcined vitriol muft be prepared from crude vitriol, where a red colour is wanted, in die feme manner as was before directed, for the fcarlet oker, which is itfelf indeed the fubftance in this cafe wanted; and will either afford, with lefs flux, a red colour in enamel, verging to the orange, or with more flux a tranfparent yellow of the warmer call. But where calcined iron is wanted, for forming more purple teints, the precipitated oker, as produced by the above means,' fhould be taken; and calcined with a ftrong fire, till it acquire the degree of purple defired : to which it may be brought by a much (hotter calcination than any other preparation of iron.
Antimony is ufed for producing a yellow colour in enamel, as well as the white before mentioned: and, indeed, is the moft ufeful, and moft ufed of any fubftance whatever for that purpofe. It is prepared only by leviga- tion; to which its texture, notwithftanding its being afemi-metal, very well fuits it. An orange colour, but not bright, may alfo be produced by antimony calcined with an equal weight or lefs of nitre, and then feparated from the fcoria that will furround it, and levigated. But as there are methods of compounding thefe colours from other neceffary preparations, there is no great occafion to have recourfe to this. There ?s a great difference in the antimony itfelf in different parcels: fome being greatly debafed by mineral fulphur; and others more free from it. That is beft which is ftriated, and has more the appearance of metal; or rather feems formed from needles laid parallel to each other; the blacker and more , fpongy, being more impregnated with crude fulphur. But the antimony is io cheap, that it is of no confequence, if the better part only of any parcel be ufed, and the reft thrown a* way : and one fide in almoft every lump is good; as, in the fufing to feparate it from the oar, the moft metalline part of courfe fubfides ;and finks to the bottom of the mafs.
Glafs of antimony is alfo ufed fometimes in enamel painting: being itfelf a fine tranlparent orange colour. But as it wants body, it has no great effedt but in compofitions. They who have occafion for this glafs may purchafe it at lb eafy a rate as renders it fcarcely worth while to prepare it themfelves; being manufactured at Venice and elfewhere in very large quantities by thole who make it their bufincfs. The only care fiiould be to chufe fuch as is not adulterated by the admixture of glafs of other kinds j which may be diftinguifhed by the force and deepnefs of the colour ; or the want of them.
Mercury is lometimes allb ufed in enamel painting ; but it requires to be prepared by fome chemical proccfs before it can be ufed. There are two preparations already pradtifed lor medicinal purpofes, which fit it alio for enamel painting the bell: of any: the produce of one is called turpeth mineral; for which we have already given the procefe p. loo: by a careful treatment of which a fine coed yellow may be produced in enamel: the other affords the red precipitate; which is a fine fear let red, but extremely tender with re;-fpeft to the fire. As this fubllance can be procured at a very moderate rate, of thole who vend it as a medicine, and requires a particu* lar nicety in the operation, I (hall wave giving any procefs for it here; elpecially as every book which treats of the chemical pharmacy contains one : and indeed the ufe of both this,- and the turpeth mineral, demand lb delicate a management of the fire, and are fo liable to have their effeft deftroyed by a fecond burning, (if, as is fo frequently die cafe it Ihould be neceflary,) that I cannot greatly recommend diem in preference to other preparations, which will anfwer the lame ends nearly as well with eafe and fafety.
Orpiment has been allb ufed in enamel for producing a yellow colour: but it is very tender with regard to the fire; and requires lo loft a flux, while at the fame dme antimony, properly managed, will fo> well fupply the place or it, that it is rarely ufed.
Powdered bricks have been alfo ufed for compounding yellow colours in enamel; but as they aft only in confequence of the oker they contain, they are certainly inferior to the prepared okers we hayc given : elpecially as they are liable to great impurities j and are harder, or require a greater force of flux, than the pure okers or calcined iron. When they are ufed, they Ihould be chofen of the reddeft colour, the fofteft and evened texture, and intirely free from all Hones or cinders. The Windlor bricks, therefore, are much the beft, that are to be procured here; as they an- fwer to the circumftances required much better than any other.
Tartar is alfb ufed. in forming enamel colours: tho’ not from any tinging quality it has in itfelf; but for its efiedt in modifying magnesia, and fome other fubftances. The crude red tartar fhould be chofen for this purpofe; and requires no other preparation, but to be freed from all impurities and well levigated.
Thefe are the feveral fubftances that are materially neceflary for compofing as well the grounds as colours and fluxes in enamel. There have been many other introduced into die practice of particular perions; and fome indeed into more general ufe; and the preparations of thofe nave been likewife greatly varied and multiplied : but what I have given are more than fuflicient for every purpofe; as all the variety of teints, with all the degrees of the attendant qualities, may be produced by a proper application of them. And the increafing unneceflarily the number of fimples and original preparations can only lead to confufion and embarraflment. But whoever acquires a moderate knowledge of the principles and fobje&s here laid before diem, may eafily proceed to examine or ufe any other colouring fubftance, which is fitted by its texture to endure the heat of vitrification.