My boiler was made using a single temperature silver solder which contained 45% silver with cadmium. It has been my experience that cadmium greatly increases the "wettting" ability of the silver solder giving better penetration in joints and more robust/even menisci. There are some basic rules to observe to achieve optimum joints those being 1) Extreme cleanliness i.e. absolutely no oil or grease on the surfaces. 2) Good tight self-holding joints with gaps no greater than 5 thousandths of an inch. 3) Use a high melting point flux. 4) DO NOT OVERHEAT the assembly - better to use an LPG burner or paraffin blow lamp rather than oxycetylene which will require much skill if overheating is to be avoided. The use of Kawool as an insulator ensures that heat is retained and cold drafts eliminated. The solder will flow towards the heat source so use the torch to draw and melt the solder accordingly. Never stick the solder rod into the flame directly but apply it to the surface of the joint away from the flame. This will ensure that the metal is up to temperature at that point and the solder will melt and flow towards the flamed area. As mentioned by another member, the re-melt temperature of of an existing silver soldered joint is about 65 degrees Centigrade higher than the original melt temperature (typically 635 C) and with an LPG burner like a Sievert, it is quite easy to revisit/re-do a joint with the same solder without re-melting the original solder. The LPG gas available in New Zealand is a mixture of Propane and Butane, less of the former usually and as the gas is consumed the Propane burns hotter than the Butane and also vaporises in preference and so if the container is not shook up regularly one can be left with only Butane and a cooler flame. Use a pickling bath to clean the job after soldering - I use citric acid which works well and fairly quickly without fear of corrosive damage to myself or my clothing.