Wiped joint

A wiped joint is a form of soldered joint used to join lead pipework.

The defining characteristic of a wiped joint is that the soldering process involves mechanically working or 'wiping' the joint. As well as heating solder and applying it to the joint, the solder is shaped into place manually, wiping it with a non-metallic tool to form a smooth-surfaced outer shape. This relies on the use of a lead-tin solder with a wide eutectic range. This is an alloy composition that when heated and melted has a large range where it is neither fully solid nor fully liquid, but is an equilibrium mixture of both solid and liquid simultaneously. The resultant 'pasty' texture and mechanical fluidity allows the joint to be wiped to shape.

The solder grade used for leadworking is plumber's solder (80% lead / 20% tin). Although this is thought of as a high melting point solder amongst lead-tin solders, the solidus is relatively constant for all of these solders and it is the liquidus which climbs from the eutectic point at Sn 63% / Pb 37%.

For pipework, the two parts are first formed to shape so as to fit closely. As lead is a ductile metal, this was easily done by hand using a variety of mallets and shaped sticks. Wooden cones could also be used to stretch the diameter of one pipe so that another of nominally the same size could now fit within it. The metal was then cleaned by scraping off the surface, then protecting it with a flux of tallow.

Heat was applied with a blowtorch and when the joint was hot, the end of a bar of solder was applied and melted onto the joint.

The molten solder gives a 'pasty' behaviour, owing to its eutectic properties and can be worked for some time before it hardens. Shaping was done with either a 'moleskin', a heat-resistant pad of cotton cloth, covered with tallow, or else a wooden stick, also greased with tallow.

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