This section is from the book "Questions And Answers On The Practice And Theory Of Sanitary Plumbing", by R. M. Starbuck. Also available from Amazon: Questions and Answers on the Practice and Theory of Sanitary Plumbing.
Heat the solder to a high temperature (red hot) and add sulphur. The sulphur combines with these impurities and causes them to rise to the surface, where they are skimmed off, after which a little rosin is thrown in.
The tin being much lighter than lead, tends to rise to the top, leaving the lead at the bottom.
The grease closes up the pores of the lead, thus preventing the bright surface from oxidizing, which action would occur in a short time, and would prevent the perfect union of solder and lead.
By adding tin.
Usually 2 1/4 or 2 1/2 inches.
A good rule is to measure one inch in each direction.
After the two ends, or the branch and the main pipe, have been fitted, the pipes are scraped until the surface which the joint is to cover is bright. Grease is then rubbed onto the bright lead. Around the outside limits of the joint, soil is either put on, paper pasted on, or other means adopted for preventing the solder from catching on the pipe beyond the joint.
Cup joints are made by flaring one end of the pipe, dropping the end of the other pipe into this flaring end, and dropping solder into the space thus formed.
An overcast joint is one made in the shape of a wiped joint, but made with a soldering iron.
Plunge a piece of dry pine or a piece of paper rolled up, into the melted solder. If it chars quickly the solder is hot enough to work, and should be watched closely from time to time, so that it may not become too hot.
First cover the work with soil, then mark out such figures as it is desired to show. After scraping and applying tallow, wipe over the surface with rich solder.
The tin being easier to melt than the lead, is naturally kept in a more fluid condition, and by continually piling on the solder around the top part of the joint, the lead by preference hangs, or sticks, while the more fluid part of the solder, which is the tin, runs to the bottom. The result is that the solder at the top of the joint becomes coarser and sets quicker, and should be wiped first. It will be noticed that the bottom of a joint is apt to be brighter than the top, and frequently a little teat will form at the lowest point.
 
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