A clip from yesterday’s work: silver soldering a habaki with the charcoal forge & fuigo box bellows. Watch the machigane area and you can see the solder begin to melt and flow around 0:40 until it is pulled out to stop the heating. The fuigo allows very precise air mix and heat control. When soldering habaki this way, some points are:
ensuring the fit is very tight (solder does not fill gaps),
cleaning the metal surfaces well to remove oxide and contaminants,
using a flux to prevent oxidation while heating (in this case borax and water),
forming a charcoal “oven” away from the direct air blast,
controlling the airflow for a reducing environment,
heating just to the flow temperature,
removing from the heat immediately,
allowing to slowly cool in air,
pickling off any remaining flux in vinegar before fitting to the blade.
Note that silver solder (more accurately known as silver brazing or silver welding) is not at all related to electronics or plumbing solder, it is made from mostly silver and copper and requires far more heat and a different flux. In this technique the filler material actually forms a new alloy with the parent metals (similar to welding). It is more difficult to achieve but the results are very different and much stronger than lead or tin based solder joints.
Learn more about the historical knifemaking process: http://islandblacksmith.ca/process/
Learn more about making habaki: http://islandblacksmith.ca/process/making-habaki/