Soldering 101 – Oxidation, Flux and Fire Scale Prevention

Nancy LT Hamilton

Last updated: 11/4/20, 7/7/18, 06/21/17, 05/01/17



Oxidation (of copper) w/o flux, Oxidation with flux

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What is the Difference, If Any Between Oxidation, Firescale (Fire Scale), and Firestain (Fire Stain)?

This is a very good question and one, of which, I’ve seen various contradictory answers.

Jewelry Materials: A Guide to Working with Common Alloys, Technical Editor: James Binnion, states that:

“Firescale and firestain are often used interchangeably, but technically can mean two different thing….Firescale is a dark gray to black scale on the surface of the silver…Firestain is a dark purple subsurface stain…”

Introduction to Precious Metals: Metallurgy for Jewelers and Silversmiths by Mark Grimwade states (on page 95):

“Oxygen in the surrounding air reacts with copper in the alloy to form black cupric oxide (CuO) on the surface and a sub-scale of reddish-grey cuprous oxide Cu20. The silver alloy will take on a blackened appearance that is known as “firestain or “fire scale”.

Deborah E. Love Jemmott states that the two terms are synonymous.  Her paper,  Firescale on Silver Instructor notes:

“Firescale (a.k.a.: firestain, stain) is that grey/black/purple stain that appears on the surface of sterling silver after it has been heated. When sterling silver is heated in the air, the oxygen in the flame and in the atmosphere attacks some of the copper alloy near the surface. This forms a copper oxide on the surface of the metal. The firescale layer deepens with prolonged heating, over heating or repeated heating. “

Charles Lewton-Brain, in his paper: Dealing with Firescale states:

“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. 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.”

So, no clarification there – either on name or spelling – firescale or fire scale.  More research for me!  Yippee! I’m going to try and get to the root of this!

Oxidation (the copper and oxygen type – there are other definitions of oxidation) is the interaction of the copper with atmospheric oxygen and heat. Oxidation is present in both firescale and firestain.

Oxidation: Cause 

Oxidation (cupric oxide) develops on metal when heat, a metal alloyed with copper, and oxygen are all present (hence the word: “oxidation”).  When we heat and pickle a metal alloyed with copper, the copper combines with oxygen and creates Cuprous Oxide Red (Cu20) and Cupric Oxide Black (Cu0).   Ever notice how blue your pickle gets after working with sterling, copper, brass, low carat golds or bronze?  The blue signifies the presence of copper in your pickle – copper that has been extracted from the sterling (or other copper alloys).

See the section, on this page, titled:  “What the Heck Are Copper Oxides” for a more detailed explanation of oxidation.

Why Did My Sterling Silver Turn White?

Well, when we heat up a copper alloy like sterling silver, we are exposing it to high levels of oxygen. Copper reacts with atmospheric oxygen and forms copper oxide –  a layer of brown-black stuff. This, unlike rust, protects the metal beneath it.

Whether you’ve annealed or soldered your metal, you’ve probably noticed, that the surface of the sterling developed a matte, white finish.  This “white stuff” (as I’ve heard it called) is simply fine silver – created when the copper is leached from the metal leaving only fine silver.  Over successive heating/pickling events, the white layers of fine silver build-up.  Eventually, these layers will be unable to oxidize, as the copper is no longer present. This is often called “bringing up the silver” or more accurately called depletion gilding.  Keep this information in mind while reading the following information.

The Relationship Between Oxidation and Finishing

So, you’ve heated your sterling silver either through soldering or annealing.  Maybe you’ve overheated it. Maybe you’ve done this a few times. You’ve pickled it after heating.  You’ve noticed that the sterling has turned white. But, you really wanted a high shine on this piece so, you pulled out your buffing wheel and some rouge.  You were almost done with the finishing BUT,  right before you could shout your victory cry,  you noticed a bruised-looking area on the metal. “What the *&%*%& is this?” You cursed.

According to Argens’ Scientists, heat in the form of friction from the polishing wheel, acts as a catalyst compelling the copper atoms to rise and re-bond with fine silver atoms on the surface. This re-bonding is evident in the micro-thin discolored surface areas on the polished silver. A second application of heat by flame will completely reverse this process, as the copper descends downward and a pure silver surface remains.

*Quote and graph from:   “Firescale: The Chameleon Effect on Sterling Silver” By Martin Ebbers. Snag News, Volume 20 Number 4,  August 2012.

So, by creating heat during the finishing process (generated by friction) of our sterling silver, the copper molecules are racing to re-alloy themselves. They tend to re-alloy in clumps creating a mottled appearance.  According to the authors, fixing this problem is to heat it again at an even higher temperature!  This will once again coat the metal with a layer of fine silver.  Yet, when you go to polish again, the same thing will happen. To keep this from happening, we need to figure out how to polish “cold”.

Avoiding the formation of fire scale/fire stain

  • Avoid excessively heating the metal.
  • Coat the entire piece in a boric acid flux or a firescale preventative like Cupronil or Firescoff. The flux coats the metal and can help to keep the copper in the metal from reacting (oxidizing) with the oxygen in the air.  But, fluxes are only effective to a certain temperature (depends on the flux).  If the metal is heated past this point, firescale can occur.  So, a combination of heat control and flux will help. Try to use a flux that has a high burn-off temperature.  The Cuprinol and Firescoff seem to work great and I’ve come to rely on them to protect my gold and silver.
  • Solder in an oxygen-free environment.  Not easy to do at home.  Maybe impossible.
  • Use a Charcoal block – the burning block of charcoal creates CO2 which removes the oxygen from the silver (to a degree).
  • Reducing the amount of time, that the metal is hot, lowering metal temperature, and avoiding repeated heating operations, will hopefully, help to reduce the depth to which the oxygen and the oxides penetrate.
  • Reducing the amount of oxygen in the flame.  Try to use a gassier flame. Although, a gassier flame will result in more “dirt”.
  • Use Argentium silver or fine silver.
  • Use of a “reducing” gas: hydrogen or carbon monoxide. Protective Atmosphere Control. Good luck with this one.

Removing Fire Scale/Fire Stain

  • Plate the piece after polishing. Of course, you’ll need plating equipment or you can send it out to be plated.
  • Don’t polish to a high shine.  Embrace the fine silver surface, have a matte surface, use patinas.  Use     radial bristle discs or other abrasives like    Abrasive Wheels and create a textured or matte surface.      A Heatless Mizzy Wheel is awesome for a matt finish too.  This is my favorite method:  the Not-So-Lazy Jeweler’s way!
  • Etch off a few layers with acid until the stain is gone. But, this method can waste the silver or gold and also destroy designs and patterns.
  • File and sand off the stain – which can take a while and may require a bit of elbow grease.   (Sanding discs).  Mechanical sanding will be a lot easier than sanding by hand, though (you can use sanding discs and a flex shaft – see my Sanding Page). Also, a destructive method as fine designs, textures, and patterns can be reduced or destroyed.
  • Electrostripping

Further Research

Updated:  January 16, 2017.

What the Heck are Copper Oxides?

There are 2 main type of copper oxides.

  • Copper/ Cuprous (I) Oxide: Cu2O.  A stable copper oxide. Reddish in color.
  • Copper/Cupric (II) Oxide: CuO. A stable copper oxide. Black in color.

During the soldering process, copper’s natural oxidizing properties are accelerated.  What forms is called:  Copper (I) Oxide – red (4 Cu + O2 â†’ 2 Cu2O) and Copper (II) Oxide – black (2 Cu + O2 â†’ 2CuO).   Copper Oxide has an entirely different set of properties from the original copper. See Bob Wilson’s explanation of the process below:

Here’s a better explanation from Bob Wilson @ Newtonask a scientist:

 “…Cupric Oxide, or CuO  is formed when oxygen in the air combines with copper atoms on the surface of metallic copper. Each copper atom on the surface donates two valence electrons to an oxygen atom, thus causing the oxygen atom to bind to the copper atom. The resulting oxide layer is relatively thin at normal temperature, and serves to protect the underlying copper atoms from further corrosion. At normal temperatures, this oxide layer looks like a slight darkening (or tarnishing) of the copper
surface. This protective oxide layer is called a “passivation layer” because it makes the copper surface “passive”, or non-reactive. There are no “free radicals” present or involved.

oxidation-on-gold-filled-14k Oxidation on 14k Gold-Filled.

oxidation-on-sterling-silver Oxidation on sterling silver – this is before pickling.

Some metals are resistant to oxidation and corrosion.  They are known as the Noble Metals:  Pure gold, pure silver, platinum, palladium, rhodium, etc.  The Noble Metals are resistant to corrosion and oxidation in air containing moisture.

WiseGeek has an article on copper oxides for further research. Also, check out: Newton – Ask a Scientist’s page on copper and oxides.

Fire Scale/Fire Stain

Fire Scale or Fire Stain is also oxidation but, it is below the surface of the metal.  Sterling silver is very prone to firescale. Firescale/stain is a gray/blue/purple coloration that forms in the top layers of the metal.  See the beginning of this article (updated 1/16/17) for the reasons that this happens.

firescale Firescale/stain.  I almost had to melt the metal to get this stain to appear.  So, another method of avoiding firescale is to not get the metal too hot.

Charles Lewton-Brain has written a detailed article on firescale, which can be found on Ganoksin.

Ways to prevent Fire Scale/Stain

  • There are many products available that coat your metal.  Firescale inhibitors work like flux.  They coat the metal with a glaze that protects the metal from interaction with oxygen in the surrounding air. cupronilCupronil (available at Rio Grande, Otto Frei, and Thunderbird Supply) and Stop-Ox (available at Rio Grande) are just two of them.  See the list of fluxes (down a few paragraphs) for other products. Also, you should read the directions on how to use these products.  You should.  Really.  No, I mean it. 
  • Don’t get the metal too hot.  Firescale/oxidation begins to form at temperatures around 1000°F (537.78°C) and extra-easy solder (the lowest melting hard solder) flows at somewhere around 1100°F (593.33°C).  A catch 22 perhaps? Saying that to achieve the firescale on the image  (of firescale) above, it took three tries and I nearly had to melt the metal to get it the stain to form.
  • Use Argentium silver (scroll down the page, in the link, for info on Argentium) or fine silver instead of sterling silver.  Probably the best advice that I can give you.  The germanium in the Argentium, coats the metal, preventing oxidation.  Period.  Fine silver doesn’t contain copper so, that is free from Firescale too.  With Argentium, it is not necessary and is in fact, not recommended to coat the entire piece with flux or an anti-firescale product.  It can interfere with germanium’s magical properties.  See Cynthia Eid’s fabulous page on soldering Argentium.

Removing firescale once you’ve obviously overheated it!

argentium-guildHere comes the fun part!  Guess what, you don’t find firescale (usually) until you are almost done with finishing – usually at the polishing stage.  Even worse is that you’ve got to remove the firescale with an abrasive, plating, or other such technique.  I sand the piece down a layer or two or, if highly detailed, use 3M’s Radial Bristle Discs with the flex shaft.  Obviously, the best way to avoid doing this is to prevent it from ever occurring!  Please read Mr. Brain’s article on removing firescale.  Buy Argentium.


Flux plays many important roles in soldering.

  •  It is responsible for reducing surface tension, allowing the solder to flow.
  • It creates a glaze, on the metal, which protects the metal from interaction with the atmosphere. 
  • Flux is also used as a temperature indicator when soldering – when either paste or liquid flux reaches solder flow point, they become translucent.
  • Flux needs to be compatible with the metal being used.  Use fluxes designed for the metal you are using.
  • Flux has a temperature ceiling.  If the heat present, surpasses the working temperature of the flux, the flux will no longer work. Liquid: 1100°F (593.33C°) – 1700°F (926.67°C)  Paste: 1100°F (593.33°C) – 1500°F (°815.56C).
  • IF you are using solder paste, check to see if the flux is included in the mixture.  If it is, you don’t need to flux. But, you might want to add additional flux to protect your sterling from firescale!

Flux is important for soldering, even if your metal doesn’t produce oxidation, like fine silver or Argentium silver.

dandix-flux Types of Flux

 THERE ARE many different types of flux.   Below is a list of just SOME of the fluxes available for hard soldering/silver soldering/brazing.  Pretty hard to pick just one.  I’ve not used Firescoff but, I’m thinking it looks interesting.  Nor have I used a gel flux – must try!

  • Aqui Flux: Liquid. Boric Acid, Borax, Ammonium Phosphate.  Similar to Prips flux. SDS
  • Batterns’ Self-pickling Liquid FluxLiquid. Sodium Tetraborate, Boric Acid, Ammonium Chloride. Gold, platinum, and silver.
  • Cupronil Anti-Firescale:  Liquid.  Firescale and flux in one. Effective temperatures: 1,100°–1,500°F (593°–816°C). Contains copper, potassium, sodium borate, and boric acid.
  •   Firescoff:   (Krohn Industries) Liquid spray. According to the manufacturer, Firescoff works up to 3000°F. Silver, gold, palladium, copper, brass, bronze, titanium, and stainless steel. No need for pickle washes off with water. Prevents fire scale. Firescoff flux is non-flammable; fluoride- and chloride-free. SDS.
  • Firescoff RH: Liquid spray. Works with the above plus, rhodium, gold-filled, or silver-filled, protecting the plated/laminated layer. MSDS.
  • Griffith Self Pickle Flux: Liquid (Grobet USA). Borax/Sodium TetraBorate mixture, Uranine 2313.
  •   Handy Flux: Paste. Fluorides, potassium, and hydroxide.  Effective temperature range: 1,100°–1,600°F (593°–871°C).  Gold, silver, brass, copper, and other non-ferrous metals. There are 2 types of Handy Flux:  One for silver, gold, brass, copper, bronze, and the other for stainless steel and nickel. The flux for stainless and nickel is called Handy Flux Type B-1.  MSDS. Not great for inhaling!!!
  • My-T-Flux: Liquid. Gold, silver, brass, bronze, nickel silver, and other non-ferrous metals; effective from 1100°-1700°F (593°-927°C). Self-pickling (although I’ve not seen it work!). Contains ammonium chlorideMSDS.
  • Griffith’s Prips Flux by Griffith. Paste. For ferrous and non-ferrous soldering. Borates and Phosphates – proprietary mix. Flux.

“This adds a sense of charm to your favorite collection.”  Perhaps my favorite flux description – ever!

  • Pro-craft® Jel-flux®:  I had to put this in here even though shipping is almost $6 for 2oz.!  The reason for its entry is this line in their Amazon ad:  ” This adds a sense of charm to your favorite collection.”  Perhaps my favorite flux description – ever!  Gel. Stays put like a paste, but will not run like a liquid, and is transparent. Jel-Flux produces fewer oxides and residues, allowing for faster pickling. No toxic fumes, biodegradable. MSDS.
  • Griffith’s Self-pickling Flux: A liquid flux specially formulated to make hard soldering on metals melting under 2000ºF as easy as soft-soldering. MSDS contact supplier.
  • Rio Ready-Flux:  Liquid. Will not bubble or pop during heating.  Gold, silver, nickel, brass, copper, and other non-ferrous metals. Self-pickling. MSDS. Note: Contains Ammonium Chloride.
  •   Magic Boric Soldering Dip: Liquid. Water-based, non-flaming Boric acid flux.  Prevents oxidation of silver during soldering. SDS – contact supplier.
  • Stop-Ox II: Liquid. Anti-Firescale. Used before flux.  Effective from 1,100°–1,500°F (593°–816°C). (Only at Rio Grande). SDS
  • Superior Flux #6 for Silver – Superior #601: This paste flux for silver brazing and soldering stays exactly where you want it and helps hold solder in place. The flux works on ferrous and non-ferrous metals as well as stainless steel and carbides. Contains no potassium bifluoride and will not release boron trifluoride gases during brazing. Gentle on metals. MSDS.  See this blog for a comparison between Superior Flux and Dandix, as well as other fluxes. SDS

So many fluxes, so little time…

Note: contrary to what I have read about the fire scale/stain preventative powers of handy-fluxPaste Flux, Cynthia Eid mentions here, that “Paste fluxes can cause firescale on both AS and SS, so they are not recommended.” Since I do not use paste flux, I have no personal experience to go on.

Powder – Various combinations of powders with fluids:

borax-coneThe old-fashioned method for creating flux (which many still use today) is to grind a borax cone, in a ceramic dish, that contains a trace of water. Jewellery Trade has made an informative video on this process. Other methods of creating flux from powders are listed below.

  • Boric acid mixed with water, boric acid dissolved in Methyl Hydrate, also know as Methylated Spirits, methyl alcohol, wood alcohol, wood naphtha or wood spirits (you can also add 30% – 50% borax to the mix).
  • Boric acid and/or Borax mixed with denatured alcohol or isopropyl or rubbing alcohol (70% –  99% isopropyl alcohol).
  • Prip’s Flux. The recipe for Prip’s Flux.

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