Hard soldering and soft soldering — invisible joints in metal

There is a reason the metal smith rarely welds: soldering is gentler on thin soft metals and gives cleaner results. In soldering two items are joined with a third metal — the solder — that melts at a lower temperature than the base material. The solder is drawn into the joint by capillary action and hardens into a strong tight connection without the items themselves melting. Well done a solder joint is strong and almost invisible.

The boundary between the two is drawn at the solder's melting point. Soft soldering uses tin-based solder that melts at low temperature (typically in the 180–250 °C range) and provides fast, but less strong joints — suitable for sealing, electronics and tin work. Hard soldering uses solder of silver or brass that melts at high temperature (over 600 °C) and provides much stronger joints. Silver soldering is the metalsmith's and silversmith's preferred method, precisely because the joint becomes both strong and almost invisible.

Solder only flows on a clean, oxide-free surface. Therefore, a flux (e.g. borax for hard soldering) is applied, which dissolves and prevents oxide during heating so the solder can wet the metal and penetrate the joint. Without flux, the solder beads up and refuses to flow. After soldering, flux leaves a glassy residue that is removed by etching. Common to all soldering is that the joining surfaces must be clean, tight-fitting and free of grease.

The key to a good joint is to heat the entire workpiece – not just the solder – to the solder's melting point. The solder flows to the hottest spot, so by directing the flame the blacksmith draws the solder the right way through the joint. If heated too locally the solder cools before it has flowed through (a cold joint); if heated too much the base material can melt or zinc can burn out of brass. On thin and thick parts the thick part is heated most so both reach temperature at the same time.

Check for a good joint

• 01Clean, tight sealing surfaces without gaps
• 02Appropriate flux applied to the entire seam
• 03The solder is seen as a thin, shiny line all the way through
• 04No porosity or cold, dull solder bead
• 05The pickling acid and rinsed after soldering to remove flux residue

“The solder flows toward the heat — master the flame, and you master the joint.”