Further firestain inquires

I have learned much about firestain and firescale from everyone’s very helpful comments in my recent queries. I have a few additional questions which I haven’t been able to find any information on, either in the archives or the internet in general. So here goes,

What temperature is considered “overheating”? does sterling overheat at hard soldering temperatures? At medium solder temperatures? The hard solder I use flows at 788 degrees Celsius, that is a bright cherry red colour and I wonder if using it “overheats” my metal thus voiding the effects of the prips flux I have been using.

Thanks for any info,



I believe that the simplest way to get an indication of temperature is to watch the color of the metal while heating…which is best seen in an area that is not brightly lit.

a very very slight dull orange color is an indication of annealed temp…

dull red metal color in a brightly lit room will really be bright cherry red in a darkened area, which is too hot.

when soldering, you will know the tempurature has been hit when the solder flows, but you want to be watching the metal to be sure that it does not overheat to a bright cherry red (not in a brightly lit area)

try taking scrap metal and heating it and watching the color of the metal
if you mark the metal with a black sharpie marker, the marker will disappear around the annealing temperature…watch the color of the metal when that is happening…

also, try heating scrap to bright cherry red (no barrier flux) then pickle and repeat a few times…then try polishing off the fine silver layer that has built up and see what the metal looks like when held over a piece of white paper…you will probably see faint purple grey shadows of firestain in the metal…then sand the metal down to see how long it takes/ how deep it is… to remove/ get below the layer of firestain.


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Firescale is forming on sterling silver at 600°C/1112°F. Sterling would show a light pink/orange colour at this temperature in a room with good light. This is an ideal annealing temperature. The depth of firescale doubles when the temperature is raised to 700°C/1292°F. The depth then triples when the temperature reaches 750°C/1382°F. The actual depth of the firestain is dependant on how long the work is held at these temperatures.


Hi Peter,

Good to know, thank you! I am going to experiment more with watching the color of the metal in a well lit room and see how I fair…

I try to wear my green lens glasses from Rio (that I use with enameling kiln) to protect me eyes, and it makes it harder to see the metal color,especially in a darkened room!

by the way, when using pripps flux, a red coating appears on the surface, around the time that I believe the metal to be achieving annealing temp…(i could be wrong…perhaps too hot?)…this red coating also seems to confuse me a bit when watching the metal…in the dark!..with my green glasses! it does come off in the pickle, and I am not getting firestain…the red color is confusing me…

I am trying to figure out best practice…I am annealing a lot lately with repousse and hope to get in alot of proper practice!


So essentially a crescendo of sadness? The hotter you go, the deeper firestain. What temperature is where the effects of prips flux (my barrier flux of choice) breaks down? Hopefully around 700 degrees…

Julie, you don’t need dark lenses when working with silver or gold. At soldering temperatures, and even when molten, neither silver nor gold produce nearly enough UV to do your eyes any damage.
Always wear dark lenses when soldering platinum. Platinum is soldered at so much higher temperatures than Ag or Au that the UV produced will damage your retinas if they are not protected. And if you melt Pt you should wear very dark welders goggles.

If you know, what temperature does prips flux degrade at? Would it be safe to heat it to the said 788 degrees WITH prips flux or has the flux already degraded leaving my piece heated way past the temperature where firestain is forming rampantly?

hi, well all i can say is that i use pripps flux, to solder and anneal, at proper temps for success, and the pripps continues to work throughout the process.

i get hot, and get in, and get out…


The problem with barrier fluxes is they tend to let you down when you need them most. It is not so much temperature that makes them fail, it is the time they are heated. If you are doing work on larger pieces and are in the “fire” for some time, or if you have a difficulty with the soldering, barrier fluxes tend to exhaust. The other problem is if you have pre-polished before final soldering. Barrier fluxes do not stick very well to a polished surface. In the 1970s in London before the problems with asbestos were known. One silversmithing workshop would mix in asbestos powder with boric powder to make a mix that would protect on pre-polished work.

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If you have never seen what firescale looks like under the surface of sterling silver look at this report. It shows cross-sectional micrographs of firestain in various alloys. The colour of the firestain changes depending which elements are being used in the alloy. Please find below a link to the report - it is a little old but the science has not changed: CATRA Firescale Report


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Hi Elliot,

ok, thank you!

I think I will continue to use my Solaris 3.0 IR green lenses…just in case…as I would like to prevent cataracts if at all possible…

I initially got them because they were recommended for my kiln enameling set…and I had seen some posts on cataracts…

I thought that James Binnions thread reply, from back in the day, (copy/pasted below) was helpful too…i think it is the IR infra red that I was trying to protect against…possible cause for cataracts…(?)


(James Binnion thread reply:)

American Welding Society and OSHA both require shade 3-5 for brazing
(our soldering) there is no differentiation for fuel type as they
all emit light that can damage your eyes when brazing.

Three sources for eye strain/damage from light radiated by torches,
the work and the brazing surface.

Infra Red, ie heat probably the biggest danger in brazing cooks the

Visible light, more a cause of eye strain than damage at lower
levels but can damage if bright enough with long enough exposure
think platinum soldering.

UV, not a big amount of UV from brazing operations and most plastic
lenses (safety glasses) are pretty good at stopping it.

Aur92 and Didymium lenses are glass lenses that are visible light
filters, they don’t much of anything about UV or Infra Red. They are
specifically designed to knock down the yellow sodium flare in hot
glass working and do a good job on the yellow flare from flux and
fire bricks.

If you solder all day every day you need to wear some form of shaded
lens (not didymium or Aur92) to knock down the infra red.If you are
soldering platinum then you need to where at least a shade 5.

James Binnion
James Binnion Metal Arts

(full thread link:)