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.