Soldering is a skill that all bench jewelers master at an early stage in their career. Although this process soon becomes second nature, sometimes itʼs good to review the basic principles involved to remind ourselves why we do certain things. Remember, an ideal soldered joint must be strong, but at the same time invisible.
When soldering is necessary, the jewelry piece must be designed with this in mind – it must be suitable for soldering. This sounds obvious but occasionally, too much attention is paid on design and not enough on how it will be manufactured, and this can lead to difficulties. If youʼre torch soldering, there must be suitable access for the flame to heat the joint area, allowing solder to flow.
The design should also be reasonably easy to finish with the tools available. Joint areas must be correctly prepared or poor joining will result. There must be a close, accurate fit between the parts being joined – solder flows into joints by capillary action, so remember this during preparation.
Make sure you clean the joint faces with either emery paper or a file to remove any burrs or rough surfaces, as well as grease, dirt, oxide and tarnish. This is essential for the solder to “wet” the metal faces being joined, allowing optimum solder flow.
Use a heat source that suits the application. Gas and compressed air is usually ok for soldering with a torch, giving good flame adjustment and movement over the complete joint area, preventing concentration in one place. If dissimilar sizes are being joined, the heat must be predominantly on the heavier piece.
Flux should be applied to the faces being joined prior to heating. It will chemically remove any residual tarnish or oxide, allowing the solder to wet the metal, and so must be fluid before any solder undergoes liquation. If insufficient flux is used, the solder will not flow successfully and may encourage the operator to overheat to achieve results. Pits in the joint and other problems will follow. You may be required to add further flux if the job has a large joint area or gap. If furnace soldering, this usually involves a protective atmosphere and so fluxing may not be necessary.
Solder may be applied by one of two general methods.
It may be pre-placed at or in the joint prior to heating and heat applied until satisfactory flow results. It may also be fed into the joint after the joint area has been bought to temperature. The heat from the joint area must always flow the solder, creating the joint. The torch flame must never be direct onto the solder. Lower melting point additions will be “burnt off”, raising the flow point of the solder and so the amount of heat required to make it flow, which may result in pitting.
Remember, solder will always flow towards the hottest part of the work piece, so keep the joint area hottest and heat localized to prevent unwanted flow onto other areas.
When copper bearing alloys (pink, yellow, green gold and sterling silver) are heated in the presence of oxygen, oxides are quickly formed. Cupric Oxide (CuO) is a black layer on the surface of the metal that can usually be dissolved in pickle. White gold produces a nickel oxide that is harder to remove, if not protected from oxidation.
Preventing Fire Scale
Flux or firecoats work by forming a coating that protects metal from oxidation.
- Borax: borax powder or sodium borate is the most common fire coat. Borax melts at 1367°F (742°C).
- Boric Acid and Alcohol: add boric acid to denatured alcohol until it stops dissolving and makes a thin paste. When soldering, the alcohol burns off, leaving a white film of borax.
- Hoover Freeflow Flux: this is a white borax-based paste which provides substantial oxide protection. This should be used on the joints to be soldered in addition to a firecoat. Becomes fluid at 1100°F (600°C).
Removing Fire Scale – Pickling
- Yellow Gold: a pickle is usually a sodium bisulfate based solution that is used to remove surface oxidation caused by heating the metal. A domestic crock pot can be used to hold and heat the pickle solution. The pickle solution works best at about 180˚F (80˚C).
- White Gold: mix an acid solution of 1 part nitric to 10 parts water. Remember, when mixing, always add acid to water.
- Sterling Silver: mix an acid solution of 1 part sulfuric to 10 parts water. Always add acid to water.
Cold Solder Joints?
Cold solder joints are caused when fusion of the solder (actually brazing alloy) and the metal has not occurred. The solder has flowed into the joint and looks soldered. However, the metal must also reach the proper temperature for correct brazing. Heat the piece of jewelry and let that heat flow the solder.
Cold solder joints happen more often with platinum because of the higher temperatures and because platinum is a poor heat transmitter. Concentrate the heat on the metal, especially if the joint area is large in mass or heavy.
Which solder do I use?
You must use the correct solder and flux for the job. Always color-match the solder used to the pieces being joined. When soldering different metals together, remember to use a solder with a flow temperature below the melt temperature of both the metals being joined. For example, when soldering a 14K palladium white gold (2030˚F) setting onto a 14K yellow gold (1550˚F) shank, use a 14K yellow solder. Refer to the chart on page 148 for the metal flow points, and pages 175 – 178 for solder flow points.
If youʼre working in white gold and use a yellow solder, expect to have a visible joint. If sizing or doing a one-stage repair, use hard solders as a preference. These are designed for use in major joining applications. Lower grade solders, particularly easy and extra-easy, are usually designed for step soldering only. The exception here can be if hollow items are to be soldered, when the lower temperatures are usually preferred.
What form of solder to use depends on application and personal preference. Sheet and wire top the popularity list, followed by paste, particularly where multiple units are soldered using a furnace. Solders can be used to fill in gaps, but there is a limit to what they can do, particularly if you are using a solder paste. These have a “binder” mixed in to keep the solder particles in suspension and can account for up to 25% of the volume of the paste, so after soldering, the actual solder will have 25% less volume. If you need to fill in gaps, use sheet or wire solder and the harder the grade, the better.
Soldering platinum used to be more straightforward. Now there are many choices of platinum such as: Pt/Ru, Pt/Ir, Pt/Co and Platinum S+ alloys. Platinum must be thought of as similar to gold soldering with many options of alloys. You must evaluate the platinum first.
Plat/10% irid and plat/5% ruth are the most popular alloys in the U.S. and can be soldered using high temperature platinum solders such as our Plumb Platinum Solders, 1700, 1600 or 1500 solders. Other 95% alloys are a little trickier.
A good way to test these alloys is to heat the metal with a torch to see if any oxidation occurs. 950 Plat/Ruth and 900 Plat/Irid will not oxidize. Plat/cobalt will oxidize blue, and Platinum S+ alloys will turn gray. This is important to know because Platinum S+ alloys and plat/cobalt have lower melting temperatures than traditional platinum alloys.
As a general rule, use Plumb Platinum Solders whenever possible. 950 Plat/Ruth and 900 Plat/Irid can be step soldered using 925 and 950 Plumb Platinum solders. Because of the lower melting points, platinum S+ alloys should be soldered using 900 Plumb Platinum solder. If a traditional platinum solder is to be used, only use 1500 solder or lower. Heat the joint thoroughly to avoid a cold solder joint.
YOU CANNOT USE PLATINUM SOLDER (1832˚F) FOR SOLDERING JEWELRY CONTAINING STONES. Diamonds can discolor or crack at temperatures of approximately 1700˚F. Use 20K White Hard solder (1615˚F) if you must keep the stones in the jewelry you are repairing.
A few tips to help reduce the likelihood of breaking prongs during soldering:
- Make sure that you have a proper fit when assembling the setting to the shank. An easy way to overheat a setting is to try to fill a gap with solder. Remember, solder is brittle by nature. Excess solder will produce a weak joint.
- Properly clean and flux the joints. If the solder will not flow, this is a good indication that the metal has oxidized. Do not try to force the solder to flow by applying more heat. Stop, re-clean and re-flux.
- Heat the shank surrounding the setting and not the head itself. Let the heat transfer to the setting from the shank. Use a soft flame. Do not overheat the prongs.
- Do not quench the ring while it is still red hot. White gold should never be quenched; this will cause brittleness. Wait at least 45-60 seconds before putting into the pickle. (See the Prong Breakage article on www.hooverandstrong.com)
It is possible to connect high karat and pure metals pieces by heating them to their melting point and allowing the surfaces to fuse together. Alloyed metals such as 10K, 14K and 18K gold are difficult to fuse because of the lack of control over melting. Fusing 22K and 24K gold produces the cleanest joint. However, 20K yellow solder is a close color match. Platinum is a poor heat transmitter, and because heat stays localized, it is easier to fuse.
So, there you have it. Soldering 101 revisited.
Copyright® 2006 Stewart Grice