It is not necessary to purify acetylene used in connection with the cutting by oxygen of metal scrap, although when the gas is employed with oxygen for welding purposes, its purification is imperative in order to obtain good results. On the other hand, where acetylene is used in connection with oxygen for higher grades of cutting on constructional work, the gas ought to be purified. Small portable generators for metal cutting are not usually provided with purifiers. Large portable and all fixed acetylene generators should be provided with purifiers, which should be charged with chemical material as recommended by the generator makers. This material as a rule should be changed after 100 cubic feet of acetylene per lb. of the chemical has passed through it.

Unpurified acetylene contains traces of lime dust, ammonia, sulphuretted hydrogen and phosphorus. In all generators the gas is allowed to bubble through water, and it is also usually filtered by passing through felt or other suitable material before entering the purifier. By these means the bulk of the three first-mentioned impurities is removed, but subsequent chemical purification is necessary to eliminate the remaining traces as well as the phosphorus. This latter impurity and sulphur are inimical to good welds, and are even liable to injuriously affect the character of the metal on the cut edges when un-purified acetylene is employed with oxygen for cutting purposes.

The subject of purification is more fully dealt with in the article on oxy-acetylene welding, page 419. The matter is of minor importance in connection with metal cutting. Where, however, acetylene generators are fitted with purifiers, the latter should always be employed even when the gas is used for metal cutting, and the maker's instructions should be followed. Purification is always advantageous, and entails very little extra expense. The degree of impurity in acetylene varies with the quality of carbide used. It can be readily tested by means of a piece of white blotting paper, moistened with a 10 per cent, solution of silver nitrate. If the paper turns black in a few seconds, when the gas is allowed to impinge upon it, considerable impurity is present. If, on the other hand, the colour of the blotting paper remains unchanged or turns only a light brown colour, the gas may be regarded as sufficiently pure, even for welding purposes.

Dissolved Acetylene From A Cylinder

Acetylene from a cylinder may be used instead of acetylene from a generator, and although its cost is of necessity higher than that of gas obtained by a generating plant direct, it has the advantage of purity, portability and simplicity.

It is employed more extensively in connection with oxy-acetylene welding than cutting, but its extreme portability renders it frequently useful for special cutting work where generators are not available, or cannot be conveniently employed.

Gas should never be drawn from a D.A. Cylinder without an acetylene regulator being interposed, but the hydraulic back pressure valve can be dispensed with and the outlet tap C of the regulator may be coupled direct to the acetylene tap H on the cutting blowpipe by means of suitable rubber tubing securely fixed.

The same general precautions should be observed for dissolved acetylene cylinders as for cylinders containing oxygen (see page 88); ascertain also that :-

1. ;Cylinders containing D.A. are conspicuously marked with the name of the manufacture and the words "acetylene compressed into porous substances exempted by Order of Secretary of State."

Each cylinder should also bear a label giving the date when it was last charged, together with the name of the firm by whom it was charged, the address of the last charging station, and the maximum pressure allowed in the cylinder.

2. ;D.A. cylinders must be painted maroon, and their valve connections must be made with left-hand screwed threads to prevent interchangeability with oxygen fittings.

3. ;See that the regulator is of the correct type for use on acetylene cylinders, i.e., painted maroon with left-hand screwed connection for fitting to the D.A. Cylinder Valves.

4. ;Acetylene regulators, being fitted with special pressure gauges suitable only for D.A. cylinders, must never be used on cylinders containing coal-gas, hydrogen or any other combustible gas.

5. ;The regulator should be adjusted to a delivery pressure not exceeding 5 lbs. per square inch when the outlet valve is opened. This will be found quite sufficient as the oxygen injector action of the cutting blowpipe will draw forward all the acetylene required. If a higher pressure than this is employed, then the flame when lighted up will probably be at some distance from the cutting blowpipe nozzle. Even at the pressure of 5 lbs. per square inch this may be the case with some cutting blowpipes. The pressure must then be reduced at the regulator by the adjustment of the milled screw C (as in the case of oxygen regulators), page 82) until this gap between the blowpipe nozzle and the flame disappears.

6. ;The amount of gas taken from a dissolved acetylene cylinder may be calculated from the loss of weight whilst in use : 1 lb. of acetylene equals approximately 13-75 cubic feet at normal temperature.

7. ;Always close cylinder valve when the cylinder is not in use.

The Oxygen Cylinder

1. In placing contracts for oxygen supplies, a guarantee from the supplier should be obtained to the effect that oxygen will only be delivered in cylinders which have been made, heat-treated, tested and filled strictly in accordance with the recommendations of the British Government Departmental Committee of 1896, or the official recommendations of 1921.

2. ;A guarantee should also be obtained to the effect that the oxygen supplied will not be less than 99 per cent, quality.

In handling oxygen cylinders the following Regulations should be carefully observed :-

3. ;See that all cylinders supplied with oxygen are painted black, and fitted with right-hand valves. Never attempt to alter the colour of cylinders or the screwed thread of valve connections.