Natural gas, propane, and hydrogen are a good fuel gas for the bench jeweler. Acetylene though often used, is a poor choice for jewelry work and should never be used with platinum. Map gas should also be avoided whenever possible.
Torch size is not only determined by the size of the object to be heated, but is also a matter of preference. A rule of thumb is to follow is to use whatever provides the required temperature in the shortest time possible while providing complete control.
Remember that heat control is always preferable to a solder stop.
The tendency of solder is to flow towards the heat. If you do not intend to have the solder flow onto a part, avoid heating it.
Avoid heating any stone directly, heat sensitive stones must be either protected or removed.
Heat tolerant stones such as diamond and corundum can withstand heat up to a limit, provided that they have not been treated in any way or quenched while hot. Thermal shock is to be avoided. Heat will reserve or damage most gemstone enhancements.
When deciding whether a stone can tolerate heat or not always error on the side of caution. Resetting a stone is always cheaper than replacing one.
Use the highest melting point solder as possible for all repairs and fabrications. When attaching parts that are frequently replaced, a medium to easy solder is preferred.
The lower the melting point of the solder the weaker the bond and generally the poorer the color match.
Gold and silver pieces should be preheated while soldering. Platinum is an exception and does not require preheating.
The surface of gold and silver must be protected from oxidation (fire scale) while being heated. A 50/50 mixture of boric acid powder and denatured alcohol provides a good protective barrier and does not interfere with solder flow.
Hard soldering fluxes are required when soldering on gold and silver. Fluxes should not be used when soldering on platinum.
Fluxes are offered in a variety of forms for different purposes. A self-pickling flux is generally not necessary when working on clean surfaces. Surfaces prone to oxidation may require the use of a self-pickling flux.
Most fluxes are formulated from a borax of fluoride base. Fluoride based fluxes generally withstand higher temperatures before breaking down.
The purpose of the flux is to promote solder flow. It is not a substitute for cleaning.
Solder stops such as yellow ochre should be thought of as a surface contaminant that interferes with solder flow. When applied to the surface of metal, it prevents solder from flowing in that area.
The torch does not flow the solder; it is the heat in the metal that causes solder to flow. The torch is used to put the heat into the metal. Heat traveling through an object is termed conduction. Heat in motion through air is called convection.
Solder will not flow on a surface that is colder than its melting point.
Soldering should always be done in a well-ventilated area. Toxic gases (such as carbon monoxide) and funes (such as cadmium, zinc, or antimony) are often produced while soldering.
Cadmium-free and antimony-free solders reduce exposure to these types of metal fumes and should be your preferred choice for gold and silver solders.