“Since the bicycle’s invention in the late 1800s the traditional way to join steel tubes to make a bicycle frame was by melting brass into a lugged joint.
Brazing, as it is known, done at a higher temperature and the resulting joint is much stronger. Early lugs were in fact pipefittings; these were heavy steel sand castings, cut square at the edges, and machined on the inside to fit the tube.
Framebuilders then started filing the lugs thinner cutting the lugs into fancy shapes to eliminate the square edge of the lug.
By the 1950s the cutting and filing of lugs became the way a framebuilder would express his art and individuality, but by the 1960s and 1970s, fancy lugwork became too costly and lugs stamped from sheet steel and welded, became available.
By the 1980s “Investment” cast lugs became available, achieved by first hand making a lug. From this “pattern” lug a simple plaster mould was made.
A lug made of wax was cast in the plaster mold; this in turn was coated in a ceramic material and fired in an oven. The firing hardened the ceramic coating and at the same time melted the wax from inside, molten steel was poured into the mold, and when cooled the ceramic mold had to be broken to remove the finished lug, hence the name, “Investment” casting. An expensive process, but the finished lug was near perfect, the tubes fit with no machining required; very little filing required from the framebuilder.
Standard lugs, bottom bracket shells, and fork crowns are made this way.”
(Source: Dave Moulton’s Blog / photo: Waterford Bikes)