EMERY.

1. - Ordinary Preparation of Emery. - The following is the manufacturers' ordinary process; the lumps of emery stone are broken up precisely after the manner of stone for repairing Macadamized roads, and into lumps of similar size. The lumps are then crushed under stampers such as are used for pounding metallic ores, and driven by water or steam power; the stampers are considered to leave the particles more angular than they would be if ground under runners, a mode sometimes employed. The coarse powder is then sifted through sieves of wire cloth, which are generally cylindrical, like the bolting cylinders of corn mills, but the sieves are covered with wire cloth, having in general about 90 to 16 wires to the inch. The following table shows the numbers of wires usually contained in the sieves, and the names of the kinds respectively produced by them: -

16. Corn emery.

24. Coarse grinding---------

36. Grinding ---------

46. Fine grinding --------

53. Super grinding ---------

60. Coarse flour emery

70. Flour ---------

80. Fine flour ---------

90. Superfine flour ---------

No. 16 sieve gives emery of about the size of mustard seed, and coarser fragments extending nearly to the size of peppercorns, are also occasionally prepared for the use of engineers.

The sieves have sometimes as many as 120 wires in the inch, the very fine sizes of emery are however more commonly sifted through lawn sieves; but the finest emery that is obtained from the manufacturers, is that which floats in the atmosphere of the stamping room, and is deposited on the beams and shelves, from which it is occasionally collected.

The manufacturers rarely or never wash the emery, which is mostly done by the glass workers, opticians, and such others as require a greater degree of precision than can be obtained by sifting.

2. - Washing Emery by Hand. - Washing-over or elutriation, as the process is called by chemists, is a valuable application of the law of gravity to the chemical, metallurgical and mechanical arts. Thus the alluvial deposits of some of the tropical rivers are washed for the separation of the particles of gold they contain. A small portion of the mud of the river is stirred in a large quantity of water contained in a broad shallow basin, the gold being several times as heavy as the earthy particles quickly subsides, and the mud which remains suspended for a long period in the water, is removed by pouring off the water from the valuable sediment.

In a similar manner the particles of emery and other powders may be separated according to their magnitudes, in a more accurate manner than can be accomplished by sieves. A portion of emery powder of uncertain size is thoroughly well mixed in a large quantity of water, as in a common wash hand basin, and at the end of 10 seconds the liquid is poured off from the sediment which has fallen down in that period; the sediment is laid aside in a separate vessel. The bulk is again stirred and poured off at 10 seconds, and this second sediment added to the first, and which process is repeated until no further sediment is deposited in the period of 10 seconds; the process requires watchfulness and a steady hand. A fresh deposit is similarly collected from the residue after a longer period of rest, say 20 seconds, until the whole quantity of emery is divided into grains of so many sizes, as may be required for the particular branch of manufacture for which it is intended; thus -

3. - Emery for the Construction of Mechanism. - The author has been for many years in the habit of employing emery of twelve degrees of fineness, part of them prepared by himself by washing over, namely:

No

. 1.

Corn emery of commerce prepared by sifting

,,

2.

Grinding

,,

,,

3.

Fine grinding

,,

,,

4.

Superfine grinding

,,

,,

5.

Deposited at the end of 2 seconds,

,,

6.

,,

5

,,

,,

7.

,,

10

,,

,,

8.

,,

20

,,

,,

9.

,,

60

,,

,,

10.

,,

3

minutes.

,,

11.

,,

15

,,

,,

12.

,,

60

,,

The emeries of the sizes 5 to 12 are preserved in glass bottles, to prevent them from becoming accidentally mixed or contaminated with foreign substances. 4. - Emery for Optical Purposes. - Mr. Ross mixes four pounds of the flour emery of commerce, with 1 ounce of powdered gum arabic, and then throws the powder into 2 gallons of clean water. He collects deposits, as above described, at the end of 10 seconds, 30 seconds, 2 minutes, 10, 20, and 60 minutes, and that which is not deposited by one hour's subsidence is thrown away as useless for grinding lenses. The use of the gum arabic, which renders the water slightly viscid, was recommended by Dr. Green for preparing red oxide of iron, for polishing specula. See Trans. Soc. of Arts. Vol. L. p. 152. 5. - Washing Emery in the Large Way. - Washing emery by hand as above explained is far too tedious for those who require very large quantities of emery, such as the manufacturers of plate glass and some others, who generally adopt the following mode: - Twelve or more cylinders of sheet copper, of the common height of about two feet, and varying from about 3, 5, 8 to 30 or 40 inches in diameter, are placed exactly level, and communicating at their upper edges, each to the next, by small troughs or channels; the largest vessel has also a waste pipe near the top.

At the commencement of the process, the cylinders are all filled to the brim with clean water, the pulverised emery is then churned up, with abundance of water in another vessel, and allowed to run into the smallest or the three inch cylinder, through a tube opposite the gutter leading to the second cylinder. The water, during its short passage across the three inch cylinder, deposits in that vessel, such of the coarsest emery as will not bear suspension for that limited time; the particles next finer, are deposited in the second or the five inch cylinder, during the somewhat longer time the mixed stream takes in passing the brim of that vessel and so on. Eventually the water forms a very languid eddy in the largest cylinder, and deposits therein the very fine particles that have remained in suspension until this period, and the water lastly escapes by the waste pipe nearly or entirely free from emery.