302 
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fo that every new artift' was' for a long time obliged to 
re-invent the method ; until at length M. Homberg, 
having once more brought the art to great perfection, 
generoufly gave the whole procefs to the world, never 
again to be forgotten or loft. 
The great objeCt of tlie operation is to take the im- 
■predion of the gem in a wery fine earth, and to prefs 
down upon this a piece of pure glafs, foftened or half 
melted at the fire, fo that the figures of the impreflion 
made in the eartli may be nicely and perfectly exprefi'ed 
upon the glafs. In general, the whole procefs much 
refembles that of the common foundery. But when it 
is brought to the trial, there occur a number of diffi¬ 
culties which were not to be forefeen, and which would 
not at all affeCt the common works of the founder. For 
his purpofe, every earth will ferve that is fine enough to 
receive the impreflion, and ductile enough not to crack 
in the drying ; thefe all ferve for the founder’s ufe, be- 
caufe themetals which they cafi; are of a nature inca- 
piable of mixing with earth, or receiving it into them, 
even if both are melted together, fo that the metal al¬ 
ways eafily and perfectly feparates itfelf from the mould ; 
but it is very dilterent in thefe caffs of glafs. They are 
compofed of a matter which differs in nothing from tliat 
of the mould, but tliat it has been run into this form by 
the agency of fire, and the other has not yet been fo run, 
but is on any occafion ready to be fo run, and will mix it- 
i'elfinfeparablywith the glafs in a large fire: confequently, 
if there be not great care taken, as well in the choice of 
the glafs as in the manner of ufing it, when the whole is 
finiflied there will be found great difficulty in feparating 
tlie glafs from the mould, and often tliiscannot be done 
without wholly dCftroying the impreflion. 
All earths run more or lefs eafily in the fire, as they 
are more or lefs mixed v.dtli faline particles in their na¬ 
tural formation. As all falts make earths run into 
glafs, and as it is neceffary to ufe an earth on this occa¬ 
fion for the making a mould, it being alfo neceffary to 
tlie perfection of the experiment that this earth fliould 
not melt or run, it is our bulinefs to fearch out for this 
purpofe fame earth which contains but very little fait. 
Of all the fpecies of earth which M. Homberg exa¬ 
mined, none proved fo much divefted of falts, or fo fit 
for the purpofe, as the common tripoli, ufed to polifii 
glafs and ftones. Of this earth there are two kinds ; 
the one reddifh, and compofed of feveral flakes or ftrata ; 
the other yellowifli, and of a finiple flruCfure. Thefe 
•stre both to be had in the fliops. The yellowifli kind is 
the heft of the two, and is commonly called {Venetian tri¬ 
poli. This receives the impreffions very beautifully ; 
.and never mixes with the glafs in the operation, which 
the red kind is apt to do. M. Homberg employed both 
kinds at once in the following manner: firfl:, powder a 
quantity of the red tripoli in an iron mortar, and fiftiiig 
it through a fine fieve fet it by for ufe ; then ferape with 
a knife a quantity of tlie yellow tripoli into a fort of 
powder, and afterwards rub it till very fine in a glafs 
mortar. The finer this p>owder is, the finer will be the 
impreflion, and tlie more accurately perfeCl tlie cart. 
The artificer might naturally fuppofe, that the befl: me¬ 
thod to obtain a purfeCl: fine powder of this earth, would 
be by wafliing it in water; but he mufl; be cautioned 
againfl: this-. There is naturally in this yellowifli tripoli 
a fort of uiicluoflty, which w'hen it is formed into a 
piould keeps its granules together, and gives the whole 
an uniform glofl’y furface : now the wafliing the. poivder 
takes away this unCtuolity; and though it renders it 
much finer, it makes ft leave a rough granulated furface 
in the mould. 
When the two tripolis are thus feparately powdered, 
tlie red kind mufl be mixed with fo much water as will 
bring it to the confluence of pafte, fo that it niay be 
moulded like a lump of dough between the fingers : 
this pafle mufl be put into a fmall crucible of a flat 
f!i ipc^ and about half an inch or a little more in depth, 
EM. . • 
and of fuch a breadth at the furface as is a little more 
than that of the flone whofe impreflion is to be taken. 
The crucible is to be nicely filled with this pafte lightly 
preffed down into it, and the furface of the pafte muft 
be ftrew’cd over with the fine pow'der of the yellow tri- 
poli not wetted. When this is done, the ftone, of which 
the impreflion is to be taken, muft be laid upon the fur¬ 
face, and prefled evenly down into the pafte with the 
finger and. thumb, fo as to make it give a ftrong and per¬ 
fect impreflion; the tripoli is then to be prelfed nicely 
even to its fides with the fingers, or with an ivory knife. 
The ftone muft be thus left a few moments, for the hu¬ 
midity of the pafte to moiften the dry pow'der of the 
yellow tripoli which is ftrewed over it : then the ftone 
is to be carefully raifed by the point of a needle fixed in 
a handle of wood ; and the crucible being turned bot¬ 
tom upwards, it will fall out, and the impreflion will re¬ 
main very beautifully and correftly taken. 
The red tripoli being the more common and the 
cheaper kind, is Itere made to fill the crucible only ta 
fave the other, v/hich alone is the fubftance fit for taking 
the impreflion. When the ftone is taken out, it mull be 
examined, to fee whether any thing be lodged in any part 
of the engraving, becaufe if there be any of the tripoli 
left, there will certainly be fo much wanting in tlie im¬ 
preflion. When the crucible and pafte are dry, a piece 
of glafs muft be chofen of a proper colour, and cut to a 
fize proper for the figure; this muft be laid over the 
mould, but in fuch a manner that it does not touch the 
figures, otherwife it would fpoil them. The crucible is 
then to be brought near the furnace by degrees, and 
gradually heated till it cannot be touched without burn¬ 
ing the fingers; then it is to be placed in the furnace un¬ 
der a muffle, furrounded with charcoal. Several of 
thefe fmall crucibles may be placed under one muffle;, 
and when they are properly difpol'ed, the aperture of the 
mutfle fliould have a large piece of burning charcoal put 
to it, and then the operator is to watch the procefs, and 
fee when the glafs begins to look bright: this is the fig- 
nal of its being fit to receive the impreflion. The cru¬ 
cible is then to be taken out of the fire ; and the hot glafs 
muft be prefl'ed down upon the mould with an iron inc 
ftrument, to make it receive the regular impreffion : as 
foon as tills is done, the crucible is to be let at the fide 
of the furnace out of the way of the w'ind, that it may 
cool gradually without breaking. When it is cold, the 
glafs is to be taken out, and its edges fliould be grated 
round with pincers, which will prevent its flying after¬ 
wards, which is an accident that fometimes happens 
when this caution has been omitted, efpecially when the 
glafs is naturally tender. The difterent colouredglalles 
are of different degrees of hardnefs, according to their 
compofition ; but the hardeft to melt are always the bell, 
for this purpofe, and this is known by a few trials. 
If it be defined to copy a ftone in relief which is natu¬ 
rally in creux, or to take one increux which is naturally 
in relief; there needs no.more tlian to take an impreflion. 
firftin wax or fulphur, and to mould tliat upon the pafte. 
of tripoli inftead of the ftone itfelf: then proceeding in 
the manner before diretfed, the procefs will have the. 
d.fired fuccefs. 
A more fimple and eafy method than the above, is by 
taking tlte calls in gypfum, or platler of Paris as it is. 
commonly called. For this purpofe, the gypfum muft. 
be finely pulverifed, and then mixed wdth clear water to 
the conlillence of thick cream. This is po.ured upon 
the face of the gem or feal of which the impreflion is 
wanted, and which mull be previouliy moiftened with 
oil to facilitate the feparation of the call; and in order, 
to confine the liquid plafter, it is'only neceflary to pin a. 
flip of oiled paper round the fides of the feal by way of 
a cap or rim. When the plafter is- dry, it is to be taken 
oft', and fet before the mouth of the furnace, in order to 
free it entirely from moifture ; then it is fit to be ufed as 
a matrix in tlie fame way as-that formed with the tripoli. 
3 earths. 
