Dr. Turner, of England, took out a pa tent in 1842 for obtaining alum by the decomposition of felspar.
It is occasionally made from the pure materials themselves. The finest pottery clay is calcined in an oven to drive off the water, and the vegetable matter combined with it ; it is then placed in a tank sunk in the ground, and to this is added sul phuric acid : the reaction is so powerful that both together soon form a boiling mixture, although no heat be applied. Water is then added, and the al lowed to settle ; the clear liquid (solution of sulphate of alumina) is then pumped up into leaden vessels, where it receives the addition of some sulphate of potash as a means ofiving the third ingredient necessary to form crystallized alum. It is, however, not yet pure, or fine in quality, and has to undergo the process of ?melting : this is meant to imply the production of an alum similar to rock alum, which derives its name from Roe cha in Syria, where it was first made. In roaehing, steam is brought to act upon the alum so as to dissolve it, and form a strong solution. This is done in a leaden vessel, from which the -solution is trans ferred to large cylindrical crystallizing vessels, where it attains the final state in which it is sent into market.
There is a peculiarity about alum which has led to an entirely new branch of manufacture. Alum is net necessarily a sulphate of alumina and potash ; in some instances soda, and in others ammonia, has been used, instead of potash, forming soda alum and ammonia alum. These bodies merely aid by facilitating the crys tallization of the sulphate of alumina, but they are of no practical service in the chief purpose to which alum is ap plied in the arts : the sulphate of alu mina is the real working agent, and if this could be obtained an a pure and solid state the alkali would in most cases be unnecessary. It happens that the iron contained in small quantity in the clay, and which would injure the alum if allowed to remain, is with difficulty re moved except by crystallization ; re cently, however, a new process for form ing a patent alum" has been adopted. In making this alum 'sulphuric acid and porcelain clay are used as before, but the clay is used in greater proportion, so as to form a mortar, which is placed in a heated trough, where it is converted into a dry earth : thence it is removed to tanks, where water dissolves it • and while here the composition is acted upon by en agent intended to remove the iron. This is yellow prussiate of potash, which by uniting with the iron forms Prussian blue ; this latter is allowed to subside, the clear liquor decanted off and boiled down to a solid residue, which is formed into cakes two inches thick,. and it is sent into the market. It is now an opaque earthy solid, differing from com mon alum by containing no potash. The
Prussian blue is collected, and is so treated as to be restored to the form which it had_previous to use, and is thus ready for a fresh quantity of crude sul phate of alumina.
Crystallized alum is composed of 1 atom of sulphate of alumina, 1 atom of sulphate of potash, 24 atoms of water Or by weight Almnina 10.82 Potash 9.94 Sulphuric. acid 83.77 Water 45.47 100.00 The ammonia alum contains more water. ALUMINA. An earth of very com mon occurrence in primitive and secon dary rocks : in the minerals felspar and :mica it is associated with silica, iron, and potash; from 'the decomposition of 'these clay is formed. It is the oxide of a metal named aluminum, and it consists of 2 equivalents of that metal united to 8 equivalents of oxygen. It can be ob tained pure by adding ammonia to a so lution of potash alum, washing the pre-. 'cipitate with warm water, and drying it: it is then hydrated, or united with water, from which it may be freed by exposure to a red heat in a crucible ; it is then a clear white powder, soft to the touch, adhesive to the tongue, and insoluble in acids. Its density varies from 2 to 4, acquiring the latter gravity after being burned ; when mixed with a small quan tity of water, it becomes doughy and plastic : if dried in this state in the air, and then heated, it cracks and shrinks from loss of water. This property is communicated to common clays, which cracks in great droughts, and to model ling clays : Wedgewood made use of it as a measure of 'heat in his pyrometer, which is now obsolete. Alumina has a great affinity for coloring principles and organic compounds, and its use in dye ing and calico-printing depends on its affinity for these substances and for woody fibre : from its double affinity it seizes upon the coloring matter and upon the tissue of the stuff, and brings them both into contact, and holds them to gether. It is on this account the basis of mordants. When ligneous fibre is not present, and alumina be added to a coloring matter of vegetable or animal origin, it unites with the latter, and car ries it down to the bottom of the vessel, leaving the liquor colorless. Colors so prepared are called a lake. Alumina freshly made is soluble in acids, and acts as a base to them ; with potash, and a few other bases, it unites, and acts as an acid. When moistened with nitrate of cobalt, and exposed to a red heat, it affords a fine blue color, by which it may be readily detected in small quantity. Alumina occurs native and pure in the sapphire, oriental ruby, topaz, and chry solite. Gibbsite and diaspore contain water united with the alumina : corun drum and emery are less pure varieties, where the alumina is mixed with a little silica and oxide of iron.