423
tures* 1 ' nearest in their process to true chemical principles,
* _. . they use no urine. The alum liquor is always purified
previous to its being 1 used. They use no alkali gene-rally, but crystallized muriate of potash ; greater eco-nomy is observed in the consumption of fuel, and theresult is a product of alum considerably larger in agiven time, and of better quality than can be producedon the old plan.
The kelp used is obtained by burning the sea wrackin kilns, at a great many places on the coast of England,Scotland , &c., but it is a very inferior alkali in an alummanufactory; it contains about 47 of soluble salts, and53 charcoal, sand, and dirt; the salts are muriate ofsoda and sulphate of soda. The refuse of soap boiler’slees are burned in a kind of oven, and sold under thename of black ashes. The composition of these ashesis about 90 soluble salts and 10 of charcoal and earths.The salts contain muriate of soda and potash, sulphateof potash, and muriate of lime and magnesia. Mr.Winter says he always found great difficulty in pro-ducing alum by the muriate of soda, and never couldform alum in any way by means of pure soda. Themuriate and sulphate of potash are the only alkalis thatcan be used to advantage in the composition of alum.
The following are the results of Mr. Winter’s experi-ments on the quantity of different alkalis required tomake 100 tons of alum ; viz.
22 tons of muriate of potash, 100 tons alum.
31 ditto black ashes ditto.
70 ditto kelp ditto.
Nicholson’s Journal, vol. xxv.
In 1828, Mr. Strachan took out a patent for an im-provement in the method of manufacturing alum, for adescription of which we refer the reader to vol. liv. of theRepertory of Arts.
f Iai >ufac-*ur e of
ha rcoal
Charcoal Works.
(600.) Charcoal, as an important article in many pro-Ce sses connected both with the Arts and Manufactures^m ay, it is presumed, be with propriety introduced into thisSe ction of the Treatise, although the place of manufacture° r preparation of it is not generally called a charcoal
■f'or common purposes the usual way of charringwood is in what are called charring pits. The wood ishrst cut into lengths of about three feet, and then piledthe ground in a circular form ; three, four, or morea p°ts, or, as they are called, cords of wood, makingwhat isjcalled a pit. These are covered with fern,
1 raw > &c., or kept down by earth or sand to support there > giving it air by vent holes as may be found neces-sary or convenient ; but this mode of charring is uncer-the* m * tS °P erat ’ on > an d in every respect defective forit • PU T° se making good charcoal, and, consequently,of o- Wtlen thus prepared very unfit for the manufacturethJ 5un P ow der, which requires all its ingredients to be of
For OSt Perfect kind -
method^ 6 * atter P ur Pose, therefore, in particular, theiron cvl’ ado P ted consists in distilling the wood inour mV i ’ not muc h unlike the method practised inPurnosp W ° r <S P ° r th e distillation of the coal. For thisWood c yhnders or retorts are employed, and the
inches in 1 6 c 7 arred ’ s cut into pieces of about nineis placert L en . g th> and then put into the cylinder, whichthe retort is° r ,r ntally - The front °P«iing or mouth ofleading- int en closed, and at the further end are pipesB t0 casks - The fire being made under the
cylinder in the proper furnace, the pyroligneous acid,attended with a large portion of earburetted hydrogengas, comes over. The gas escapes while the liquordescends through the pipes, and is collected in the casks,and the fire is kept up till no more liquor or gas will comeover. This, it is evident, is a more eligible method thanthe former, indeed the only proper one, and the differ-ence in the strength of powder made from the two sortsof charcoal is such, that since the Government powderhas been made with it, the charges of powder for thesame piece of ordnance are said to have been reducedone-third. We suspect, however, that some of this re-duction is due to the more accurate boring of the gun,the gauging of the ball, and the resulting reduction ofwindage. It should be remarked, that the wood beforecharring has its bark taken off; and for this reason it isbest felled in the Summer season, when the sap is up,and will admit of the bark being more easily and cleanlyremoved. One reason for taking off the bark is, thatpowder, into which the burned bark enters, is full ofsparks, which is, of course, both injurious arid dan-gerous.
The wood made use. of is commonly elder, willow, orblack-dog wood, and these are said to be very superiorfor the purpose in the old way of charring, althoughthere are some differences in opinion on this point; butit is generally admitted, that in distilling in cylinders asabove described, the particular nature of the wood is byno means important.
Indigo Works.
(601.) Although the preparation of indigo can have noclaim to be considered as a British process, yet as it is soimportant to the art of dyeing, printing, &c. in thisCountry, and is principally derived from our foreigndependencies, and, moreover, the operation itself beingvery interesting, we do not hesitate to include indigoworks amongst the general list of those which relate tothe preparation of various substances of a similar classemployed in the Arts and Manufactures of Great Britain .
Indigo, the most important of the vegetable coloursused in dyeing, unlike logwood, madder, and most othercolours, is extracted from the tree or shrub whichcontains it, before it arrives in this Country. It isusually imported into Great Britain from the West Indies and America ; it is also made in the East Indies,where its cultivation has latterly been much encouraged.
Indigo, or its basis, is contained in the Isatis txnctonawhich grows in temperate climates, and in some legu-minous plants of warmer latitudes, particularly variousspecies of the Indigofera. The plants from which ourindigo is usually produced are, 1. Indigofera anil, a largeAmerican plant, which affords indigo of good quality.2. Indigofera tinctoria, a Chinese and Indian plant,which has been carried to America ; this is less hardybut more productive than the preceding. 3. Indigo-fera disperma, which affords the Guatimala indigo ; itsproduce is uncommonly fine, but not abundant. 4.Indigofera argentia, which produces the indigo bastardof the French ; and, 5. Nerium tinctoria, from whichthe East Indian indigo is now produced.
(602.) History of indigo. Considerable uncertaintyseems to exist about the early history of indigo, whichprobably arises from the circumstance of all Indian pro-ductions having been brought at that time over laud, or3 i 2
Mechanical
Processes.