Before attempting to light the gas the operator must be sure that all the air is exhausted from the tubes. Otherwise the flame will go back and explode in the tubes or fire trap. To be sure of this the beginner must test the gas. A handy test tube can be made by capping one end of a piece of ½-inch pipe, which should be about 6 inches long. To test the gas, first open wide the gas cock M on the generator, Fig. 1. Then open the gas cock f on the mixing fork and let the gas displace the air in the tubes, which it will do in about one minute. Then invert the test tube, Fig. 20, and hold it over the blow pipe tip for a moment until the gas has displaced the air in the tube. Then quickly place your thumb over the opening of the test tube, which will keep the gas from escaping. Close the gas cock f on the mixing fork, then take the test tube to one side away from the generator, still keeping it inverted, and bring it close to a lighted match or candle. It will light with a pop, and if it is free from air it will burn quietly down in the tube until the gas is exhausted. Continue to test the gas in this manner until it burns as described, when it may be safely lit at the jet without fear of its burning back. This precaution is necessary only after opening the generator for some purpose.
Fig. 20.—Method of Testing Gas.
Regulating Volume and Pressure.
This generator evolves gas under a greater pressure than can be used on most work, and for this reason the flame will at first be long, noisy and unsteady, as shown in A, Fig. 21, but, as there are two cocks, the volume and pressure can be regulated to the requirements of the work at hand. Now, to note the peculiarities of this flame, we will close the gas cock f on the mixing fork until the flame is about 3 inches long. It will be of a pale reddish color and will burn steadily. The inner flame is not as yet very well defined. Then open the air cock g slowly, and when sufficient air has been admitted the flame will be seen to shoot out suddenly and then shorten to about 1¾ or 2 inches in length. It will be smooth, compact, and will have the appearance of darting rapidly. If the correct quantity of air has been admitted the inner flame, as shown in B, Fig. 21, will then be plainly seen, and its apex, which is the point of greatest heat, will be blue. This inner flame is known as the nonoxidizing flame, and is the flame with which the fusing is done.
Fig. 21.—Flames Under Different Pressures.
The outer flame will change to a bluish color. Its temperature is low, and its effect on the lead is to coat the metal with a heavy blue oxide, under which the lead runs but does not unite. To demonstrate this, bring the point of the inner or nonoxidizing flame to bear on a piece of sheet lead. It will fuse bright and clean and will have a circle of gray oxide around it. Then quickly remove the flame and the spot will remain bright. Now, again bring the flame to bear on the same piece of lead, keeping the point of the inner flame at least ¾ inch away from the lead. It will melt and flow together, but will be covered with a coat of gray oxide and the union will not be perfect. Slowly withdraw the flame, and before the flame is entirely removed the spot will be heavily coated with a thick blue oxide, under which the lead will not unite. C, in Fig. 21, shows the appearance of the flame when too much air has been admitted and it is on the point of going out from lack of gas.
The proper way to use the gas is to open wide the gas cock M on the generator, and do any regulating of the flame with the gas cock f on the mixing fork. These cocks should have pieces of heavy wire brazed or soldered lengthwise of the handles, Fig. 16, h and i, so as to form lever handles. This will allow the gas and air cocks to be closed or opened by gently tapping the levers h and i, which is the only way that a slight variation can be had, for if you try to regulate them with the finger you will constantly open or close them too much, and the result is that in adding air too much is always admitted, which will blow out the flame, making it necessary to turn off the air and light the jet again, and many times this operation will have to be repeated before the flame is correctly adjusted.
It must be remembered to always turn on and light the gas before admitting any air, and when through with the flame the air must be turned off first, then the gas. If this operation is reversed an explosive mixture of gas and air would form in the tube and would spoil the tube, if nothing worse.
Study the Flame Well.
The beginner should study the flame until perfectly familiar with the color and form of the proper flame. One of the greatest troubles that the beginner will have with the blow pipe is the inability to regulate the flame to the requirements of the work. For instance: A flame that would work nicely on 12-pound sheets would burn holes in 4-pound sheets before you had time to touch the lead with the inner flame. For that reason three different sizes of tips should be used. On a 2-pound sheet the smallest, or 1-32, tip should be used, and the flame before reducing should not be longer than ¾ inch, and when reduced the inner flame can hardly be distinguished, but you can easily tell when it touches the lead by the metal fusing bright. If it is desired to fuse 12-pound sheets the 2-32-inch tip should be substituted, and it would be found necessary to have the jet of gas about 3 inches long, which, when reduced, would be about 2 inches long and would show the inner flame very distinctly.
The only way to determine the size of the flame necessary is by experimenting with it. It will also come with experience. The flame should be reduced to a size that will not melt the lead as soon as it touches it. Rather, it should be in such condition that the lead would have to be heated first and let the fusing come gradually. In that way it can be determined just what sized drop is required, and also plenty of time is allowed to place it just where it is wanted—particularly on upright seams and imperatively on inverted seams.
It is not necessary to be so particular on horizontal seams, as on seams in that position you are assisted by gravity. The lead drop that is melted from the upper lap cannot do otherwise than unite with the under lap. It must be remembered that in starting a seam you have cold lead to fuse, and after the first drop is started the lead in its vicinity will be heated almost to the melting point, and you will probably be surprised to see the lead run at the approach of the flame for the next application.
Do Not Hurry.
The point to be taught here is that you must not attempt to hurry this work or holes will surely be burned in the sheets, which oftentimes makes difficult work to patch. The old adage, “haste makes waste,” can well be applied to lead burning. Sufficient time must be allowed for one drop to set before attempting to place the next drop. Time spent on practicing at the bench is time well spent, as many little details that cannot be brought to the beginner's attention here will be learned in that way and stored in his mind for future application.
