ULTRAMARINE (_Outremer_, Fr.; _Ultramarins_, Germ.); is a beautiful blue
pigment obtained from the variegated blue mineral, called lazulite
(_lapis lazuli_), by the following process:--Grind the stone to
fragments, rejecting all the colourless bits, calcine at a red heat,
quench in water, and then grind to an impalpable powder along with
water, in a paint-mill, (see PAINTS, GRINDING OF,) or with a porphyry
slab and muller. The paste being dried, is to be rubbed to powder, and
passed through a silk sieve. 100 parts of it are to be mixed with 40 of
rosin, 20 of white wax, 25 of linseed oil, and 15 of Burgundy pitch,
previously melted together. This resinous compound is to be poured hot
into cold water; kneaded well first with two spatulas, then with the
hands, and then formed into one or more small rolls. Some persons
prescribe leaving these pieces in the water during 15 days, and then
kneading them in it, whereby they give out the blue pigment, apparently
because the ultramarine matter adheres less strongly than the _gangue_,
or merely siliceous matter of the mineral, to the resinous paste. MM.
Clement and Desormes, who were the first to divine the true nature of
this pigment, think that the soda contained in the lazulite, uniting
with the oil and the rosin, forms a species of soap, which serves to
wash out the colouring-matter. If it should not separate readily, water
heated to about 150° F. should be had recourse to. When the water is
sufficiently charged with blue colour, it is poured off and replaced by
fresh water; and the kneading and change of water are repeated till the
whole of the colour is extracted. Others knead the mixed resinous mass
under a slender stream of water, which runs off with the colour into a
large earthen pan. The first waters afford, by rest, a deposit of the
finest ultramarine; the second, a somewhat inferior article, and so on.
Each must be washed afterwards with several more waters, before they
acquire the highest quality of tone; then dried separately, and freed
from any adhering particles of the pitchy compound by digestion in
alcohol. The remainder of the mass being melted with oil, and kneaded in
water containing a little soda or potash, yields an inferior pigment,
called _ultramarine ashes_. The best _ultramarine_ is a splendid blue
pigment, which works well with oil, and is not liable to change by time.
Its price in Italy was five guineas the ounce, a few years ago, but it
is now greatly reduced.
The blue colour of _lazulite_ had been always ascribed to iron, till MM.
Clement and Desormes, by a most careful analysis, showed it to consist
of--silica, 34; alumina, 33; sulphur, 3; soda, 22; and that the iron,
carbonate of lime, &c. were accidental ingredients, essential neither to
the mineral, nor to the pigment made from it. By another analyst, the
constituents are said to be--silica, 44; alumina, 35; and soda, 21; and
by a third, potassa was found instead of soda, showing shades of
difference in the composition of the stone.
Till a few years ago, every attempt failed to make ultramarine
artificially. At length, in 1828, M. Guimet resolved the problem, guided
by the analysis of MM. Clement and Desormes, and by an observation of M.
Tassaert, that a blue substance like ultramarine was occasionally
produced on the sandstone hearths of his reverberatory soda furnaces. Of
M. Guimet’s finest pigment I received a bottle several years ago, from
my friend M. Merimée, secretary of the _Ecole de Beaux Arts_, which has
been found by artists little, if any, inferior to the lazulite
ultramarine. M. Guimet sells it at 60 francs per pound French,--which is
little more than two guineas the English pound. He has kept his process
secret. But M. Gmelin, of Tübingen, has published a prescription for
making it; which consists in enclosing carefully in a Hessian crucible a
mixture of 2 parts of sulphur, and 1 of dry carbonate of soda, heating
them gradually to redness till the mass fuses, and then sprinkling into
it by degrees another mixture, of silicate of soda, and aluminate of
soda; the first containing 72 parts of silica, and the second 70 parts
of alumina. The crucible must be exposed after this for an hour to the
fire. The ultramarine will be formed by this time; only it contains a
little sulphur, which can be separated by means of water. M. Persoz,
professor of chemistry at Strasbourg, has likewise succeeded in making
an ultramarine, of perhaps still better quality than that of M. Guimet.
Lastly, M. Robiquet has announced, that it is easy to form ultramarine,
by heating to redness a proper mixture of kaolin (China clay), sulphur,
and carbonate of soda. It would therefore appear, from the preceding
details, that ultramarine may be regarded as a compound of silicate of
alumina, silicate of soda, with sulphuret of sodium; and that to the
reaction of the last constituent upon the former two, it owes its
colour.
UMBER, is a massive mineral; fracture large and flat; conchoidal in the
great, very fine earthy in the small; dull; colour, liver,
chestnut,--dark yellowish brown; opaque; does not soil, but writes;
adheres strongly to the tongue, feels a little rough and meagre, and is
very soft; specific gravity 2·2. It occurs in beds with brown jasper in
the Island of Cyprus, and is used by painters as a brown colour, and to
make varnish dry quickly.
URANIUM, is a rare metal, first discovered by Klaproth, in the black
mineral called _pechblende_, found in a mine near Johann-Georgen-Stadt,
in Saxony, and which is a sulphuret of uranium. A double phosphate of
uranium and copper, called _green uranite_, and _uran mica_, occurs in
Cornwall. It has been reduced to the metallic state by various devices,
but it has hardly the appearance of metal to the naked eye, and from the
rarity of its ores is not likely to be of any importance in the arts.
URAO, is the native name of a sesquicarbonate of soda found at the
bottom of certain lakes in Mexico, especially to the north of Zacatecas,
and in several other provinces; also in South America at Columbia, 48
English miles from Merida.