Updated: Feb 21, 2022
Fire scale or fire stain 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. 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. Fire scale 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 to it and 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 fire scale to occur. In alloys of lower silver content this effect is limited by the fine-grained eutectic structure that occurs in them. Fire scale occurs as the silver in the alloy conducts the oxygen to the interior of the metal where it forms Cu2O. Sterling silver is known for its tendency to form fire scale during soldering and annealing operations. A technique I use to protect my jewelry from fire scale is a solution of Boric Acid and Denatured Alcohol. Caution: Because of the fire hazard, this approach is not recommended to use in your home. To use this technique, you need a safe work environment.
Hazard note: Alcohol fumes are toxic. Make sure you have adequate ventilation. The most common additive to denatured alcohol is 5 to 10 percent methanol. Methanol is highly toxic when consumed orally. Studies have reported deaths in people consuming as little as 0.5 ounces of 40-percent methanol.
Make sure you have a fire extinguisher close in case of fire.
Always wash your hands and the silver. I use Dawn dish soap to remove contaminants.
The water-based approach is much safer and should be used if possible. In the Denatured 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. Only mix small amounts at a time in an open mouth container. Mix the amount that you need to immerse the piece. I usually mix 1 Tablespoon of boric acid to 3/4 cup of Denatured Alcohol. Mix thoroughly by stirring, then dip the piece. Place the piece on the charcoal block to dry or you can ignite the alcohol to burn off the Denatured Alcohol. The Denatured Alcohol burns with a greenish flame leaving an even layer of the powder over the surface of the metal.
If by chance the Denatured 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. Keep a fire extinguisher close by just in case it’s needed
Small Studio Approaches There are then several approaches to fire scale. One can remove it by abrasion (polishing,
sand-blasting, filing, sanding) and remove all metal to below the level of the fire scale. 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.
The most basic thing you can do in a small workshop to limit oxidation is to use a slightly reducing flame (more gas than oxygen) which produces a local reducing atmosphere on the surface where the unburned gas present seeks to bond with available oxygen thus preventing it from entering the surface of the metal. You don’t have control over the fuel to air mixture with naturally aspirated torches. That’s why I use Propane and Oxygen torch. This subject will be addressed in a future blog, “Torch Selection and Why”
This is one of the reasons that silversmiths traditionally solder on charcoal blocks. It’s a way of obtaining a reducing atmosphere. The charcoal burns forming CO2 and removes oxygen from the vicinity of the metal surface. It’s 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 so at best one can in this way limit the degree to which fire scale occurs. Usually a boric acid based mixture similar to those described above is the most appropriate solution for a small workshop. Denatured 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 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 or the flame as it leaves the object turns yellow (a temperature indicator of about 800oF) followed by a pickling in an acid solution that attacks copper oxides preferentially to silver and copper thus removing the copper from the surface and leaving the fine silver behind. Many such pickling solutions are used, from vinegar and salt to the jeweller’s standard pickle, Sparex®, which is sodium bisulfate. The product that I use is called PH Down manufactured by Dow chemical. It’s used to lower the PH of swimming pools and It's readily available everywhere.
The effect is to give the surface a dead white covering which although 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 high polished surface is desired the metal is prepolished (and usually one can finish the polishing and eliminate the fire scale completely during this stage-the heating and pickling may be extra work) and then the surface is depletion silvered as described above. Then it may be very lightly buffed 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 fire scale beneath) or burnished in a burnishing tumbler with steel shot. Like an industrial electroplating it can be worn off and the fire scale beneath appear again in the worn areas.
The best technique is to make sure there are no blemishes on the pieces before you start soldering. This will take you more time, but you will find it’s really a time saver because you can use a tumbler to burnish and bulk finish your pieces. A tumbler cannot remove blemishes or scratches.
A 50/50 mixture of sodium bisulfate pickle and hydrogen peroxide solution is referred to as super pickle, It’s a stronger mixture which can be used to remove copper.