Gold is a soft metal. This means gold rings commonly become bent or misshapen. Even day-to-day wear can do this, since the heat of the hand is sufficient to soften the gold further. Fortunately, gold's malleability also means that bent gold rings are relatively easy to fix. If the ring is severely squashed it may be cracked, or may become so as it is pulled back into shape. In such cases, repair the split first, then reshape.
Use pliers to gently pull out crushed sections of a badly flattened ring. Take ring into the mouth of the pliers. Move in a circular manner around the ring, squeezing it into shape. Check for breaks or cracks.
File around cracks to remove any old solder. Pry the gold band apart at the break, sufficient to insert the flat file into the gap. File broken ends, removing rough edges.
Use vice grips to close up the break. Inspect under the magnifying glass. File broken ends again if the join is not smooth and tight.
Clean ring thoroughly -- see resource. Check the hallmark inside the ring to discover whether it is 10-, 14- or 18-carat.
Using tweezers, dip ring into borax flux. Heat ring briefly with the acetylene torch until the flux burns, creating a coating that will protect it from heat damage and oxidation.
Cut a 1mm square portion of solder using the scissors. Avoid problems with the ring melting before the solder does by using solder softer than the gold in the ring. Choose 14 carat solder for best results -- this solder melts at the lowest temperature.
Hold the ring in vice grips, broken side facing out. Either preheat the solder portion slightly with the torch, or add a drop of flux to the break in the ring. Use tweezers, pick or moistened cocktail stick to lift the sliver of solder onto the break.
Heat using the acetylene torch until solder melts and flows into the join -- this should take only seconds. Keep the flame low and in motion -- do not overheat. Turn off torch. Leave ring to cool.
File inside the ring with half-round file, smoothing the joint. Repeat on outside. Finish with fine sandpaper.
Slip ring onto the mandrel or dowel. Warm in the hands for thin rings; use the torch to warm heavy rings. (References 1, 6)
Gently tap warmed ring down over the mandrel with the mallet until circular. Remove and buff to a shine.
It may help to work in a slightly darkened room during soldering -- if the ring glows red, it is time to reduce the heat. Filing down the teeth of the vice grips helps avoid scratching the ring.
Gold rings with stones are more complex. Heat may crack or discolour gemstones. Remove stones from setting before soldering or use a specialised soldering block with a "heat sink."