Firescale or firestain is a reddish purple toned bloom or stain that appears on silver/copper alloys such as sterling silver when they are heated in the presence of oxygen. It even occurs in gold alloys with high copper contents. As it generally appears in the form of blotchy patches following abrasive polishing, it is seen as a blemish which destroys the clear reflectivity of finished silver surfaces. It is usually observed best when the silver object is placed on a piece of white paper. Sometimes it is chosen as a surface finish in order to emphasize texture as when a silversmith like Lois Betteridge uses firescale to accentuate the visual depth of the hammer marks on a surface by polishing off the tops of the resulting texture leaving them a lighter gray than the reddish tones of the firescale in the recesses.
Firescale is caused by oxygen combining with copper present in the silver-copper alloy. A sterling silver alloy contains 925 parts out of a thousand parts silver, and 75 parts out of a thousand parts copper. When the alloy is exposed to oxygen in the air at higher temperatures during annealing or soldering procedures, the copper at the surface is converted to Cu2O (cuprous oxide) which has a reddish color, then to cupric oxide CuO which is black. It is, however, not just the metal at the very surface that is affected but deep inside the metal as well. Silver has the ability to absorb oxygen at high temperatures and conduct the oxygen to the interior of the metal itself where it can bond with copper atoms present thus causing deep firescale to occur. Alloys, such as reticulation silver, contain 800/1000ths parts fine silver and are especially susceptible to oxidation. In alloys of lower silver content this effect is limited by the fine grained eutectic structure that occurs in them. In low gold content silver/gold/copper alloys, firescale also occurs as the silver in the alloy conducts the oxygen to the interior of the metal where it forms Cu2O. Sterling silver is the standard alloy in North America and is well known for it's tendency to form firescale during soldering and annealing operations.
Industry deals with the problem of firescale in a number of ways, most of which eliminate oxygen from the surface of the metal during a heating process. The solutions used include heating in a hydrogen or ammonia atmospheres where copper oxides can actually be converted back to copper. Production line work is sometimes placed in titanium jigs on a moving conveyer belt which enters a hydrogen, nitrogen, or ammonia gas atmosphere where heating for soldering occurs. The object then cools down in the same atmosphere and emerges at the other end soldered and ready to be removed form the jig-with no firescale.
If a minimal number of solderings are to be done, a protective coating is placed on the work so oxygen cannot reach the surface during heating. The protective coatings are usually based on boric acid which is deposited evenly over the work. One production shop I worked in Toronto used a large wire French fry basket in which all the items to be soldered were placed. This was held suspended in a rapidly boiling solution of boric acid and borax mixed with water. After the items were heated, the basket was placed over a a large flame to dry. The pieces were then dumped out on the soldering board and set up to solder. I noticed in the boric acid approach, where the borax based soldering flux is used on the same piece, firescale is likely to occur.
Another approach used in industry is to make a solution of boric acid, borax, and solvent alcohol. Because of the potential for accident this approach is not recommended. The water based approach is much safer and should be used if possible. In the alcohol based procedure a container with a lid is chosen. Unbreakable ceramic or heavy plastic is recommended as the danger of fire by accidental breakage of a glass container is very high. A halfway mark is made on one side of the container and the other side of the container is marked in thirds. Boric acid powder or crystals (from the drug store) is poured in until the halfway mark is reached. Then borax powder is added until the top third mark is reached. These proportions would be good for the water based approach as well. Solvent alcohol is added until the container is almost full. The mixture is shaken to thoroughly mix it and then the work is dipped in it, quickly placed on the soldering block, and set alight. The alcohol burns with a greenish flame and the heat of its burning deposits an even, thin protective layer over the surface of the metal. Again, where regular flux has to be applied for soldering parts together, firescale will have a tendency to appear. If one is just annealing a single time, as when a highly polished object is repaired, the polish will be almost completely retained on the metal. One might expect that one could prepolish pieces before assembly and perform multiple solderings without affecting the polish but this is not really possible as the firescale preventative is not a hundred percent effective, and after several heatings there tends to be an accumulation of firescale such that the polishing time required is the same as when no preventative was used. In the industry, therefore, this approach is almost always used only for repairs or when there are only one or two soldering steps involved.
If by chance the alcohol in the container catches fire the solution is very simple: calmly put the lid back on which extinguishes the flames. A panicked reaction to flames in the container which knocks it over can result in a severe fire. A further hazard note on an alcohol based firescale preventative solution is that alcohol fumes are toxic. While this would make little difference for the occasional jewelry scale object, larger objects treated in this manner give off a lot of fumes. A teacher of mine who was working on a large liturgical piece which was regularly swabbed with a similar solution went to the doctor for his annual checkup and was asked if he had a drinking problem (which he strenuously denied). It turned out that the alcohol fumes were affecting his liver, and the addition of an alcohol fume mask to his workshop (and one would hope better ventilation) solved the problem.
A common solution chosen for production pieces is to electroplate the pre-polished surfaces with silver or with rhodium which has a hard, dead chrome-like color. It also has an unfortunate tendency when heated, (as during a repair) to burn to a hard and nasty black color which requires polishing to remove it. Silver plating is fine for smaller items like brooches, but on surfaces where wear is a problem the firescale will eventually emerge from beneath the plate again. For silver, polishing each individual piece to remove firescale would require too much labor and dramatically raise the unit cost to the manufacturer. This does not mean that hand made work must be competitive with such mass-produced items, as it often addresses a slightly different market sector.
Small Studio Approaches
There are then several approaches to preventing firescale. One can remove it by abrasion (polishing, sand-blasting, filing, emerying) and remove all metal to below the level of the firescale. This is time-consuming and can be wasteful of material and if polishing is hurried and careless can lead to a loss of detail or crispness in the piece. If polishing is chosen as a solution, the best approach is to begin with bristle brushes on the polishing machine and use a lot of tripoli or other cutting compound, as it is the compound that does the work, not the brush. This can, with practice, be controlled by guiding the path of the bristles so that little detail and a minimum of metal is removed. Polishing demands a high degree of concentration to understand it's control factors. Traditional polishing apprenticeships lasted from three to five years; not that this is the best way to learn, only that there is obviously a lot to learn. Brushes combined with thoughtful use of other buffs can offer sharp edges and minimally disturbed detail even though the firescale has been removed.
While some people have built appropriate gas atmosphere kilns, this is beyond the interests and skills of most individuals in the field. In medieval times and today in many parts of the world, work is soldered and heated on a charcoal forge into which air is blown through tubes or pumped from bellows or with a motorized fan (for example a vacuum cleaner) which causes the heat to rise by supplying oxygen. This system produces a highly reducing (low oxygen or oxygen devouring) atmosphere which limits oxidation somewhat. The most basic thing you can do in a small workshop to limit oxidation is to use a slightly gassy flame (more gas than air) which produces a local reducing atmosphere on the surface where the unburned gas that is present seeks to bond with available oxygen, thus preventing it from entering the surface of the metal. This is one of the reasons that goldsmiths in the west traditionally soldered on charcoal blocks. It was a way of obtaining a reducing atmosphere--the charcoal present burns forming CO2 and removes oxygen from the vicinity of the metal surface. Its burning also heats the metal and provides a high ambient temperature which then allows easy local heating of areas for soldering. Charcoal blocks are, however, expensive and it is difficult to completely envelop a piece with such a reducing flame during a heating operation. Usually a boric acid based mixture similar to those described above is the most appropriate solution for a small workshop. Alcohol based solutions work well but are a severe fire hazard and their fumes may present problems. A water based solution is safer.
A choice often used in art schools is to bring up the fine silver, a misleading term for depletion silvering. This involves the repeated oxidation of surface or near surface copper in the alloy by heating until the surface discolors (a temperature indicator of about 800ºF) followed by a pickling in an acid solution that attacks copper oxides leaving the fine silver behind. Many such pickling solutions are used, from vinegar and salt, to the standard Sparex jeweler's pickle, which is sodium bisulfate and is often sold as swimming pool acid. The effect is to give the surface a dead white covering which, though attractive, tarnishes in time and is therefore not an easily maintained permanent finish. The surface may be brass brushed with a little soapy water in between each heating and acid pickling sequence and this renders a smooth, bright, satin finish. If a highly polished surface is desired, the metal is prepolished and then the surface is depletion silvered as described above. One can finish the polishing and eliminate the firescale completely during this stage; the heating and pickling may be extra work. Then the piece may be very lightly polished with a new buff with only a faint trace of rouge on it (so as not to polish off the fine silver and expose the firescale beneath) or burnished in a burnishing tumbler with steel shot. As in industrial electroplating, the plate will eventually wear off, exposing the firescale beneath.
There are then several approaches
to firescale prevention: avoidance (creating oxygen-free conditions
during heating), abrasive removal (removal by polishing or other
abrasive techniques), chemical removal (removal of the surface of the
metal by bombing or stripping--very
covering up (plating or depletion silvering).
Personally, I deal with
sterling's potential for firescale in two ways, depending upon the
potential wear on the surface. If possible I brass brush my work
which has a depletion silver surface, choosing that as my primary
finish, then burnishing the highlights. If a silver piece is subject
to wear (a ring for example) then it must be polished and I usually
use bristle brushes, and then larger cotton buffs to smooth the
surface, and finally, if necessary, flex shaft brushes and buffs.
Most jewelry objects can be polished to remove firescale in 5-10
minutes using bristle brushes and lots of compound. Remember, it is
the compound that does the work, not the buff. If it is a single
soldering or just a few, then I will use a boric acid firescale
preventative solution on the work. It depends on which method looks
like the shortest in time and trouble for that particular piece.
Lewton-Brain, Box 1624, Ste M, Calgary, Alberta, T2P 1N3, Canada