C02 G L A S 
ill order that the workmen may have eafy acesfs to the 
'vorking-flues, and that the heat may be moderated for 
working, I form certain openings in the faid building, 
which are to be clofed by iron doors, or otherwife, 
M henever extreme heat is required, but which are opened 
by taking the fame down at the time of working, and 
tiiereby permit the external air tb enter between the 
iurnace and the furrounding edifice, lb as to reduce the 
draught to the ordinary flandard or force. The figure 
and dimenfions of tlie faid furrounding edifice or build¬ 
ing are fufceptible of confiderablc variation, according 
to the circumftances of the work itfelf, and the judge¬ 
ment of the operator ; who muft be fuppofed to polfefs 
fufiicient Ikill to perform a work of this kind in a proper 
manner ; but in general I advife a conical figure, of con- 
liderable elevation, for the external building, having its 
orifice or chimney at the top, and its bafe of fuch a dia¬ 
meter, as that its internal face may be lefs than one foot 
diftant from the neared projebling part of the curvature 
of the furnace, with fide-apertures, to be clofed as afore- 
faid, oppofite the working-flues; for the fafe and per¬ 
fect condruftion of which, the (kill and precautions iil'ii- 
;illy employed in works of this nature will be fully fuf- 
ficient, without further indrublion.” 
Of coloured GLASS. 
It is probable that there was no great interval be¬ 
tween the difeovery of the art of making glafs, and that 
of giving it diderent colours. When the fubdance of 
which, it is formed contains, by accident, any metallic 
particles, the glafs ad'umes fome tint; and this liappens 
oftener than Is vvidied ; nay, a confiderablc degree of care 
and forefight is neceflary to produce glafs perfectly co- 
lourlels. From the different colours, therefore, w hich 
glals acquired of itfelf, it was eafy to conceive the idea 
of giving it the tinge of fome precious done, which is 
noiiiing more than a glafs formed by nature. This art, 
in ancient times, was undoubtedly carried to a very great 
extent. Proofs of this may be found in Pliny, who, 
among others, mentions artificial hyacinths, fapphires, 
and that black glafs which approached very near to the 
obiidian done, and which in more than one place he 
gemnice vitrea. See the article Gem, p. 293, of this 
volume. 
I'hcglafs-houfesat Alexandria were celebrated among 
the ancients for the fkill and ingenuity of the v.'orkmen 
employed in them. From tliele the Romans, who did 
not acquire a knowledge of that art till a late period, 
procured for a long time all their glafs ware. From the 
learned author of Rechcrches fur les Egyptiens et ks Chinois, 
it appears that coloured glafs was made even in thofe 
early ages. The emperor Adrian received as a pre- 
feiit from an Egyptian pried, feveral glafs cups wliich 
fparkled with colours ot every kind, and wliich, as codly 
wares, he ordered to be ufed only on grand fedivals. 
Strabo tells us, that a glafs-maker in Alexandria in¬ 
formed him that an eartli was found in Egypt, w’ithout 
which the valuable coloured glals could not be made. 
What materials the ancients tiled for colouring glafs, 
has not been communicated to us by any ot their writers. 
It is, however, certain tliat metallic calces only can be 
employed for that purpofe, becaiile thefe pigments rea¬ 
dily withftand the iieat of the furnace j and it is liighly 
probable that ferruginous earth, if not the foie, was at 
ieaft the principal, lubdance by which not only all diades 
of red, violet, and yellow, but even aTilue colour, could 
be coinnuiiiicated, as profellbr Guielin has Ihewn. Re- 
fpettiiig the red, there is the lels doubt, as, at prefent, 
lonietimes an artificial, and fometimes a natural, iron 
ochre, is often employed for that purpofe. For com¬ 
mon works this is fufficient; but when pure clear glafs, 
coloured drongly iliioughoul with a beautiful lively 
red, tree from flaws and in tbmewliat large pieces, is re¬ 
quired, iron is not fit, becaufe its colour, by the conti¬ 
nued Jieal neceflary for making glafs, either difappears, 
or becomes dirty and opaque. In the feventeenth cen¬ 
tury, fome artids in Germany fird fell upon the method 
of employing gold, indead of iron, and of thereby mak¬ 
ing artificial rubies, which, when they were well fet, 
could deceive the eye of a connoifleur, unlefs lie tried 
them with a diamond, ora file. The ufual method was 
to dilfolve the gold in aqua regia, and to precipitate it 
by a folution of tin, w hen it atfumed the form of a pur¬ 
ple-coloured powder. This fubdance, which mud be 
mixed wfitli the bed flint, is called the precipitate or 
gold calx of Callius, gold-purple, or mineral purple. 
In the year 1606, when Libavius publilhed his Al¬ 
chemy, the art of making ruby glafs appears to have 
been unknown. He indeed quotes an old receipt for 
making rubies; and conjettures, that becaufe the real 
dones of tlie fame name are found in the neigliboiirhood 
of gold mines, they may liave acquired their colour from 
that metal; and that, by means of art, glafs might be 
coloured by a folution of gold. The later chemids, 
however, and particularly Achard, found no traces of 
gold, but of iron, in that precious done. TJie idea 
which Libavius conceived from a fulfe deduction, has 
neverthelefs been confirmed by experience. Neri, who 
lived almod at the fame time as Libavius, was better 
acquainted with the gold-purple, though his receipt is 
very defedfive. According to his directions, the gold 
folution mud be evaporated, and the remains fuft'ered 
to continue over the fire until they become of a purple 
colour. One may readily believe that this colour will 
be produced ; but glafs will Icarcely be coloured equally 
through by this powder; and, wluit is worfe, fome of 
the gold particles will he apt to diew themfelves in it. 
artid, in the feventeenth century, underdood bet¬ 
ter the life and preparation of gold-purple than John 
Kunkel, wlio, after being ennobled by Charles XI. king 
of Sweden, alFumed the name of Lewendiern. He him- 
felf tells us, that he made artificial rubies in great 
abundance, and fold them, by weight, at a high price. 
He fays, he made for the elector of Cologne a cup ef 
ruby glafs, weighing not lefs than twenty-four pounds, 
which was a full inch in thicknefs, and of an equal beau¬ 
tiful colour throughout. He employed himlelf mod on 
this art after he engaged in the fervice of Frederic- 
'VVilliam eledtor of Brandenburg, in 1679. At that time 
he was inl'pedtor of the glafs-houft's at Potldam; and, 
in order that the art of making coloured glafs might be 
brought to perfedtion, the eledtor expended a very large 
fum. A cup with a cover, of this manufadture, is dill 
preferved as a curiofity at Berlin. 
Painting on glafs and in enamel, and the preparation 
of coloured materials for mol’aic work, may, in certain 
refpedts, be confidered as branches of the art of colour¬ 
ing glafs ; and in all thel'e a beautiful red is the mod 
difficult, the deared, and the fcarced. When one care¬ 
fully examines the old madcr-pieccs of dained glafs, one 
finds either that the panes have on one lide a tranfparent 
red varnilh burnt into tlicm, or that the pieces which 
arc llained through and through, are thinner than thofe 
coloured in tlie other manner. It is, therefore, extreme¬ 
ly probable, that the old artids, as tliey did not know 
how to give to thick pieces a beautiful tranfparent red 
colour, employed only iron, or manganele, which pig¬ 
ment, as already oblcrved, ealily becomes in a drong 
heal blackilh and muddy. limimel painters, liowever, 
were for a long time obliged to be contented with it. A 
red colour in mofaic work is attended with lefs difficulty, 
becaui'e no traniparency, nay ratlicr opacity, is required. 
At Rome thole pieces are valued mod, whicli have the 
beautiful Ihining red colour of the fined fealing-wax. Wg 
are told that I'uch pieces were made only out of a kind of 
copper drols ; and at prelent there are feveral tirtids in 
that city who prepare thele materials, but they are not 
able to give thc»u the ricli and relplenderU colours of 
the ancients. 
When coloured glafs is to be blown, or run into fquares, 
from 
