Silver Soldering in the Shop
With the parts for the barrel of my 24-PDR Coehorn Mortar M-1838 built and ready to be joined together, the best method is through the use of 45% silver solder.
Over the years I have used this method many time over, and there is some skill required to achieve good silver soldered joints in the assembly of the parts using this method.
Also regardless of the material to be joined together whether it be brass or steel, the joints to be soldered need to be scrupulously cleaned before attempting the soldering.
The integrity of the silver soldered joint depends on successful capillary action, the process by which the silver solder will flow, once the temperature is drawn through the entire joint. The strength of the completed joint is directly related to the space between the two components being assembled. Besides joint clearance, capillary action is also affected by surface finish of the components being bonded. Unlike soft soldering, a mechanical metallurgical action happens between the components being bonded and the silver filler material. The filler at flow temperature actually permeates the surface of the components and creates a very strong joint.
I always use a aircraft style oxyacetylene torch which is sized especially for silver soldering in my shop. This small Purox torch is especially suited for this type of work, as the temperature that 45% silver solder flows at, is approaching 1200 degrees F. I did have to use my largest tip for this job, as the size of the two parts being joined required a fair amount of heat to get them up to the temperature required to flow solder.
The flux used in silver soldering will tell you when the components being heated, and the joint in particular is ready to flow solder. At 212 degrees F the water in the flux boils off. As the temperature rises to 600 degrees F, the flux becomes puffy and starts a dry bubbling. At 800 degrees F, the flux flattens against the surface of the components with a milky appearance. At 1100 degrees F, the flux liquefies to clear appearance, exposing clean metal underneath. At this point, your almost ready to flow the solder, that requires 1200 degrees F to melt. Once this temperature is reached, the solder melts, flashing through the joint, and your done.
Just so you know, I had just laid down the coil of 45% SS in the photo, as the parts had reached the critical temperature for soldering, and if you expand the photo and look close, you can see where the solder had flashed through the joint between the barrel and the rear trunnion upon applying the solder.
The insert photo shows how it looks after the cleanup.