.

3. Oxygen and Illuminating Gas. This is a very satisfactory method,
and one that has become very popular. In this method it is absolutely
necessary to have a flash back tank (Fig. 79) in the gas line to
prevent the oxygen from backing up into the gas line and making a
highly explosive mixture which will cause a violent explosion that may
wreck the entire shop.

[Fig. 79 Flash-back tank for lead burning outfit]

To make such a trap, any strong walled vessel may be used, as shown in
Figure 79. A six to eight inch length of four inch pipe with caps
screwed over the ends will make a good trap. One of the caps should
have a 1/2 inch hole drilled and tapped with a pipe thread at the
center. This cap should also have two holes drilled and tapped to take
a 1/4 inch pipe, these holes being near the inner wall of the large
pipe, and diametrically opposite one another.

Into one of these holes screw a short length of 1/4 inch pipe so Fig.
79. Flash-Back Tank for Lead Burning Outfit that it comes flush with
the inner face of the cap. This pipe should lead to the burning outfit.

Into the other small hole screw a length of 1/4 inch pipe so that its
lower end comes within 1/2 inch of the bottom of the trap. This pipe
is to be connected to the illuminating gas supply.

To use the trap, fill within one inch of top with water, and screw a
1/2 inch plug into the center hole. All connections should be airtight.

4. Acetylene and Compressed Air. The acetylene is bought in tanks, and
the air compressed by a pump.

5. Hydrogen and Compressed Air. This is the method that was very
popular several years ago, but is not used to any extent at present
because of the development of the first three methods. A special torch
and low pressure air supply give a very satisfactory flame.

6. Wood Alcohol Torch. A hand torch with a double jet burner gives a
very clean, nonoxidizing flame. The flame is not as sharp as the
oxygen flame, and the torch is not easily handled without the use of
burning collars and moulds. The torch has the advantage of being
small, light and portable. A joint may be burned without removing the
battery from the car.

7. Gasoline Torch. A double jet gasoline torch may be used, provided
collars or moulds are used to prevent the lead from running off. The
torch gives a broad flame which heats the parts very slowly, and the
work cannot be controlled as easily as in the preceding methods.

[Fig. 80 Carbon lead bu