Nancy LT Hamilton
Last updated: 2/4/25, 11/4/20, 7/7/18, 06/21/17, 05/01/17
Contents
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 I’ve seen various contradictory answers to it.
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 to get to the root of this!
Oxidation (the copper and oxygen type – there are other definitions of oxidation) is the interaction of copper with atmospheric oxygen and heat. Oxidation is present in both firescale and firestain. It only occurs in metals that contain copper.
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 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 gold, or bronze? The blue signifies the presence of copper in your pickle.
What the Heck are Copper Oxides?
There are two main types 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 → 2 Cu2O) and Copper (II) Oxide – black (2 Cu + O2 → 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 more scientific explanation from Bob Wilson @ Newton – ask 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 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. Also, we need to include Argentium here.
WiseGeek has an article on copper oxides for further research. Also, check out Newton – Ask a Scientist’s page on copper and oxides.
Firescale/Firestain
Firescale or Firestain is also oxidation, but this form of oxidation is below the metal’s surface. Sterling silver is very prone to firescale. Firescale/stain is a gray/blue/purple coloration that forms in the top layers of the metal.
You really want a high shine on this piece, so you pull out your buffing wheel and some rouge. You are almost done with the finishing BUT, right before you shout “success!”, you notice a bruised-looking area on the metal. “What the *&%*%& is this?” You curse. Welcome to the weird world of fire scale/fire stain! It’s a depressing place!
Firescale/stain. I almost had to melt the metal to get this stain to appear.
Charles Lewton-Brain has written a detailed article on firescale, found on Ganoksin.
The Relationship Between Oxidation and Finishing
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.
Avoiding the formation of fire scale/fire stain
- The best way to avoid fire scale/fire stain is to use Argentium silver.
- Use Argentium silver 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. Fine silver doesn’t contain copper, so it is free from Firescale, too. Higher karat golds will be less likely to form fire stain. With Argentium, it is not necessary and is, in fact, not recommended to coat the entire piece with flux or an anti-fire scale product. It can interfere with germanium’s magical properties. See Cynthia Eid’s fabulous page on soldering Argentium.
- Use medium silver solder instead of hard, or better yet, purchase Argentium solder. Your seams won’t tarnish, and the color match will be closer.
- 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 keep the copper from reacting (oxidizing) with the oxygen in the air. However, fluxes are only effective at a specific temperature (depending on the flux). If the metal is heated past this point, fire scale/fire stain 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.
- Use a Charcoal block – the burning block of charcoal creates CO2, which depletes the oxygen from the silver (to a degree).
- Reducing the amount of time the metal is hot, lowering the metal’s temperature, and avoiding repeated heating operations will help reduce the depth to which the oxides penetrate.
“‘Come in hot and get out fast!”
- Reduce the amount of oxygen in the flame. Try to use a gassier flame. However, a gassier flame will result in more “dirt.”
- Use of a “reducing” gas like hydrogen. To learn more about hydrogen torches, see this Rio Grande video.
. Hydrogen torch at Rio Grande (about $2,500.00 US)
- Solder in an oxygen-free environment. Good luck with this one. You’ll need a space suit or dive helmet. (Kidding – don’t do this!)
- 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 above (the photo of fire stain), it took three tries, and I nearly had to melt the metal to get the stain to form. Follow my adage: Come in hot and get out fast!
Removing Fire Scale/Fire Stain (if you must)
- You can “bring up the silver” or depletion guild the metal. Bringing up layers of pure silver. Of course, if you polish it, you will remove the fine silver. You could just leave it white. Don’t do this with a ring – the fine silver coating will scratch and eventually wear off. Maybe a gentle sweep with a Pro Polish Pad?
- Plate it! 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 silver surface, create a matte surface, and/or use patinas. Use
radial bristle discs or other abrasives, like these
Abrasive Wheels, to create a textured or matte surface.
A Heatless Mizzy Wheel is awesome for a matte finish, too. These tools are some of my favorite methods: the Not-So-Lazy Jeweler’s way!
- Etch off a few layers with acid until the stain is gone. However, this method can waste silver or gold and 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 much easier than hand sanding, though. You can use sanding discs with a flex shaft (see my Sanding Page) or a buffing machine. Abrasives are destructive and can destroy delicate designs, textures, and patterns.
- Electrostripping
- Hide it! Solder a stamping or decorative element on top of the stain!
Further Research
- Check out Martinus’s website, Martinus Master Goldsmith, for ways to prevent firescale.
- Dealing with Fire Scale by Charles Lewton Brain at Ganoksin.
- Ganoksin Orchid, Discussion on Reduction Atmosphere in a Kiln.
Why Did My Sterling Silver Turn White?
When we heat a copper alloy like sterling silver, we expose it to high oxygen levels. Copper reacts with atmospheric oxygen and forms copper oxide – a layer of brown-black stuff. This, unlike rust, protects the metal beneath it. But, sometimes, something else happens, and we end up with white silver. This “white stuff” (as I’ve heard it called) is simply fine silver – created when the copper is leached from the metal, leaving only the silver. Over successive heating/pickling events, the white layers of fine silver build-up. Eventually, these layers will not oxidize, as the copper is no longer present. This is often called “bringing up the silver” or, more accurately, depletion gilding. This layer of pure silver is pretty thin, so take care to protect the surface.
Flux
Flux plays many vital roles in soldering.
- It is responsible for reducing surface tension, allowing the solder to flow.
- It creates a glaze on the metal, protecting it from interaction with the atmosphere and oxygen.
- Flux is also used as a temperature indicator while soldering – when paste or liquid flux reaches the solder flow point, it becomes 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) – 1700°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 flux to protect your sterling from firescale!
Flux is essential for soldering, even if your metal doesn’t produce oxidation, like fine silver or Argentium silver.
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 have used Firescoff, and it works great but is pricey! I have not tried gel flux. Currently, I use Pepe Tools smart flux.
- Aqui Flux: Liquid. Boric Acid, Borax, Ammonium Phosphate. Similar to Prips flux. SDS
- Batterns’ Self-pickling Liquid Flux: Liquid. 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. Some like this better than the pricier Firescoff.
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. There is no need to pickle as it brushes off with water. Prevents fire scale. Firescoff flux is non-flammable, fluoride- and chloride-free. SDS. You can really see the difference when using it. Unfortunately, it’s super pricey!
- Firescoff RH: Liquid spray. Works with the above plus rhodium, gold-filled, or silver-filled, protecting the plated/laminated layer. MSDS. (The link is to the 4 oz size which Rio Grande does not sell. You will go through this stuff.
- 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 two types of Handy Flux: One for silver, gold, brass, copper, and 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 chloride. MSDS.
- 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! (from Pro-craft Jel-flux)
- 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. It 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. I have never used this product, so I cannot support or deny their claims.
- Griffith’s Self-pickling Flux: A liquid flux formulated to make hard soldering on metals melting under 2000ºF as easy as soft-soldering. MSDS contact supplier.
- Rio Ready-Flux: Liquid. It 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 precisely where you want it and helps hold the solder in place. The flux works on ferrous and non-ferrous metals, stainless steel, and carbides. It contains no potassium bifluoride (an etchant for glass) and will not release boron trifluoride gases (toxic) during brazing. Gentle on metals. MSDS. See this blog for a comparison between Superior Flux, Dandix, and other fluxes. SDS
- I now use
So many fluxes, so little time…
Note: contrary to what I have read about the fire scale/stain preventative powers of Paste Flux, Cynthia Eid mentions here that “Paste fluxes can cause firescale on both AS and SS, so they are not recommended.” I don’t have an opinion since I do not use paste flux (anymore).
Powder – Various combinations of powders with fluids.
Another method for creating flux is to grind a borax cone (or 20 Mule Team Borax) in a ceramic dish that contains a trace of water (or alcohol). Jewellery Trade has made an informative video on this process. Other methods of creating flux from powders are listed below. You might find this Ganoksin discussion interesting, too! Why do I keep leading you off of my website?
- Boric acid mixed with water, boric acid dissolved in Methyl Hydrate, also known 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.
Related Videos
-
- How to Set Up a Torch – have a torch, need to set it up? Here’s how.
- Soldering 101, part one, and Soldering 101, part two
- All About Solder – You need to understand what you are working with!
- Beginning Jewelry Project: Making and Soldering a Pendant. A beginning soldering project to get you started.
- Beginning Jewelry Project: Making and Soldering a Pearl Ring. Another beginning soldering project.
- Flat Square Edges on Metal – Important techniques for creating square edges like for ring shanks, tubing, settings, etc. – pretty much any two pieces that you want to solder together!
- Getting Ready to Solder – Important steps before you solder.
- Identify Wire Solder – Mark your solder before confusion reigns!
- Jewelry Studio Safety – Extremely important information that every jeweler should know! Don’t risk your life or your health! Know the dangers of metal dust? If not, don’t sand anything – yet!
- Creating Consistently Sized Spheres – how do you make perfectly round spheres? You might need to know!
- How to Anneal Silver Sheet is an important skill; otherwise, you can damage your metal and work twice as hard!
- Annealing Wire – Want broken wire? How about molten wire? Need your wire soft and bendable? These tips will help you to avoid these problems and work with ease!
- DIY Fume Extractor for the Studio or Shop – One of the BIG ONES for studio safety. Learn to make your own!
- How to Make a Bezel and Set a Cabochon Part One – Want to solder thin material that is too thick? Want to set a stone or two? Learn how to create and solder bezels.
- How to Make a Bezel and Set a Cabochon Part Two – Ditto from above.
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Soldering Jewelry: How to solder settings, bails, and wire. – We solder dissimilar shapes and sizes of metal together all the time. Want to stop melting your settings or bails? Can you solder wire without melting it?
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Soldering Jewelry – Sweat, Flush, or Applique Soldering – How to solder two pieces of metal on top of each other. Learn tricks for successfully joining dissimilar sizes and how to apply solder.
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My YouTube Soldering Playlist – A list of all my soldering videos on YouTube.
Related Web Pages
- About Solder – Learn all about the material you use.
- Acetylene, Torch, Tanks, Safety – A huge page with so much more than info on Acetylene! Learn all about torches, soldering, and how to protect yourself!
- Charts – Soldering related charts. This includes things like annealing temps, compressed gas valve sizes, the temperature at which your gas burns, and the melting points of your metal. Also, there are wire gauge charts, the millimeter to fractions and inches charts, and the drill bits to wire gauge charts. Lots of information!
- Cleaning Metal – nice to know if you plan on soldering anything!
- On Pickle, Acid, Crock Pots, and Baking Soda – How to remove the schmutz left from soldering, how to make your own pickle, how to use pickle, and how to neutralize pickle. Tons of info!
- Oxidation, Flux, and Fire scale – Why does oxidation occur? Why do you keep getting fire scale? How do you get rid of it? Learn the whys of what is happening when you solder and the solutions.
- The 4 Steps for Successful Soldering – The 4 steps will help you to achieve soldering success!
- Identifying Wire Solder – How to mark your solder so that you always know what type it is.
- Jewelry Tools – Harbor Freight – Cheap tools for the studio!
- Miter Cutting Vise and Jig: Having a hard time squaring up the ends of your ring shanks? Check out this tool!
- Q&A: Firescale/Firestain – See what others have had problems with and find the solutions!
- Q&A: Annealing – How long to hold your annealing temps? Kiln annealing.
- Q&A: Wire Questions. Balling up wire, tapering wire, work hardening wire, straightening wire, and more!
- Recipes: They aren’t just for cookin’ anymore!
- Pickle Recipe – make your own pickle
- Prip’s Flux Recipe – make your own flux.
- Removing Broken Drill Bits From Your Metal -snapped your drill bit and can’t get it out? Here’s how to remove broken drill bits.
- Removing Copper Flashing, i.e., How to remove the copper coating you might get from pickling, and also how to remove copper from brass or bronze that comes to the metal’s surface after soldering.
- Torches – Learn more about the different gases for soldering and the torches that go with them.
- Wire and Sheet Metal
- What Torch to Buy: Trying to figure out what you need to make a fire in your studio? Check out this information before you buy!
- Soldering Questions – One of the most asked after subjects. Many of my web pages have been inspired by soldering issues and questions.
- Soldering in a Nutshell – my list of basic necessities for soldering.
- Torch/Gas Questions – Portable vs. regular torches, torch problems, butane torches, water torches, setting up a torch safely, buying torches.