THE PROBLEM

It’s a problem that plagues the jewelry industry:  a lack of uniform standards for designating karat gold solders. Typically, a manufacturer will independently develop a family of gold solders and compare the melting points of each. The manufacturer will assign a designation to the solder with the lowest melting point (i.e., extra easy, ultra easy, extra soft, etc.), and then assign the opposite designation to the solder with the highest melting point (i.e., hard, seamless, weld, etc.). The remaining solders will be slotted according to their relative relationships.

However, this system creates problems for the bench jeweler who chooses solders from different manufacturers or uses different karat or color combinations while step soldering. Such problems are usually re-solved through trial and error, but this is often at the cost of damaged items.

In this article, I am suggesting a better system for designating solders, based on a universal temperature scale that everyone can understand. Such a system would enable bench jewelers to order a solder from any manufacturer and know that its liquidus temperature would fall within a specific range for a specific designation.

The first step in developing such a scale is to establish the relationship between solder and metal substrates. Currently, designations such as “hard” or “soft” refer only to how the liquidus temperatures (flow points) differ within a particular family of solders. But knowing a solder’s flow point and how it relates to the melting range of the metal is critical; it will determine whether two items can be successfully joined by that solder. Since we want the solder to melt and flow before the metal substrates do, it is most logical to base the scale on the temperature difference between the solidus temperature of the karat metal and the liquidus temperature of the solder. Based on this, I developed the following framework for the proposed scale:

• Solders that have liquidus temperatures within 25°F (14°C) of the melting point of the karated metal would be designated as weld solders. The weld designation would indicate some melting of the substrate material, involving mutual solubility between the solder and the substrate materials, with a high degree of diffusion at the interface. The joint could not be separated by reheating. Weld solders should be used when a permanent joint with maximum strength is required.

• Solders that have flow points 26°F to 95°F (15°C to 53°C) below the melting point of the karated metal would be designated as hard solders. The hard solder designation would indicate that there is mutual solubility between the solder and substrate materials, involving a moderate degree of diffusion of material across the liquid/solid interface. Depending on where in the temperature range the flow point falls, separation of the joint through reheating may be extremely difficult.

In the World Gold Council’s Hand-book on Soldering and Other Joining Techniques, metallurgical consultant Mark Grimwade of Middlesex, England, states that “ideally there should be a difference of at least 50°C between the temperature at which the solder is completely molten and will flow and the solidus temperature of the parent metal, in order to avoid incipient melting of the component pieces.” This guideline can be used to determine when hard solder joints could be separated, if necessary, by reheating. (This temperature relationship is far more crucial with torch soldering than in oven soldering applications, where the temperature can be tightly controlled and more evenly applied.) Hard solders should be used where a permanent joint is required and incipient melting of the substrate can be tolerated as long as it is kept to a minimum.

• Solders with flow points 96°F to 145°F (54°C to 80°C) below the melting point of the karated metal would be designated as medium solders. The medium designation would indicate a strong mechanical bond, with a minimal degree of diffusion on the atomic scale across the liquid/solid interface. This joint could be successfully separated through reheating.

• Solders with flow points 146°F to 195°F (81°C to 108°C) below the melting point of the karated metal would be designated as easy solders. The easy designation would indicate a moderately strong mechanical bond, with negligible diffusion of material across the liquid/solid interface. This joint could be separated readily through reheating.

• Any solders with flow points 196°F (109°C) or more below the melting point of the karated metal would be designated as extra, ultra, or super easy solder. The extra easy designation would indicate a bond with the least amount of diffusion across the liquid/solid interface and that could be separated easily by reheating.

Although temperature is the most easily recognized factor in soldering, it is important to acknowledge that temperature alone is not the only consideration in the behavior of solder with the substrate. In practical application, the wetting and spreading characteristics of the solder cannot be overlooked. As David Jacobson of Buckinghamshire Chilterns University College in High Wycombe, UK, points out, “The wetting and spreading characteristics at the temperature of use (which may be prescribed by the solidus temperature of the parent material) are the key issues here and they depend on composition no less than on temperature.”

It’s equally important to acknowledge that the wetting and spreading characteristics of solder are affected by the composition of the substrate as well, since karated metals can include a range of alloys (such as palladium or nickel in white golds) formulated to produce variations within a karat/color range.

Introduction | The Procedure >>