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The Absolute Last Final Word on Anhydrous Borax Flux


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Short version: Anhydrous borax is a great flux for forge welding. It can be purchased in bulk or produced by melting regular borax (e.g., 40 Mule Team), letting it cool, and grinding it up. Drying regular borax in an oven isn't enough.

Long version: Borax is a great flux for forge welding. When sprinkled on the surfaces to be welded, it melts, dissolves the oxides that form during heating, and carries them out of the weld under the force of the hammer blows. It is cheap and easily obtained as a laundry booster. The disadvantage of regular borax as it comes out of the box is that it contains a fair amount of water. When that water evaporates, it makes the borax bubble and foam, and sometimes even fall off the workpiece before it melts completely. One solution to this problem is to use anhydrous borax.

First, science! Borax is one of three compounds:

  • anhydrous sodium tetraborate, Na₂B₄O₇
  • sodium tetraborate pentahydrate, Na₂B₄O₇·5H₂O
  • sodium tetraborate decahydrate, Na₂B₄O₇·10H₂O

Note that all three compounds are sodium tetraborate; the only difference is the presence and amount of water contained within the borax crystals. Solid forms of both the pentahydrate and the decahydrate occur when the water evaporates from aqueous borax solutions. As the water evaporates out and borax crystals form, a certain amount of water gets trapped within the crystals; this is known as "crystal water" or "water of crystallization". All three forms are hygroscopic, which means that they will absorb some degree of moisture from their environment. However, that moisture will remain on the surface of the borax particles and will not change the amount of crystal water. In this case, the borax can be dried by heating above 212ºF/100ºC (e.g., in the oven) to evaporate out the excess water.

By way of comparison, think of a bucket of sand. If it's completely dry, it's loose and flows easily. If some water is added, the sand sticks together and holds its shape. As the sand dries out, the clumps fall apart again.

Now, there's a BIG difference between dry borax and anhydrous borax. Drying borax as described removes only the water on the surface of the borax crystals and not the crystal water. Anhydrous borax, on the other hand, is formed by the removal of the crystal water. The only way to do this is to heat the regular borax until its crystals melt, at which point the crystal water is released and boils off.

So, why is this important? Well, in order for the borax to work as a flux, it has to melt and flow over the entire surface, which happens around 1770ºF/966ºC (this is why we apply borax at an orange heat or hotter). The problem is that when the pentahydrate and decahydrate forms melt and release their crystal water, that water flashes to vapor and makes the borax foam and bubble. That makes it harder for the borax to wet the weld surfaces and can (as noted above) make the flux fall off the workpiece, especially if the borax is applied heavily and traps water vapor under it. This can be minimized by adding flux slowly or making multiple applications of flux, but can be wasteful and messy.

Since borax is cheap, many smiths may find this problem insignificant and not worth bothering about. That's perfectly acceptable. It should be noted, however, that melted borax is quite caustic and can seriously corrode firebrick and other refractory, especially in gas forges. Minimizing borax mess is a good way to help keep your forge in good shape.

Anhydrous borax, on the other hand, melts quickly and smoothly with minimal foaming and minimal waste.  It is commercially available, but usually in bulk quantities. It takes some time, effort, and fuel to produce in the shop, but the superior results can certainly be worth the investment.

To make true anhydrous borax, we need to heat the borax beyond the point where the moisture evaporates from the surface of the crystals, until the crystals themselves melt and release the crystal water. When the resulting goo cools, it hardens into a solid moisture-free mass. When this is ground up into fine particles (for example, with a mortar and pestle or a coffee grinder), those particles have no crystal water and are truly anhydrous. While this powder is hygroscopic and may absorb some moisture from the surrounding air, it will never regain its crystal water (unless dissolved in water and allowed to dry, in which case it would be either the pentahydrate or the decahydrate). This can be used as-is or as an ingredient in other flux mixtures with boric acid powder, charcoal dust, iron filing, etc.

IFI member jlpservicesinc has a video showing what that melting process looks like. She notes that it takes about 3.5 pints of regular borax to produce one pint of anhydrous.

It should be noted that because the melted borax is dissolving any oxidation or other impurities from the surface of the pan, the resulting anhydrous borax is not chemically pure. This is not a problem with forge welding, but may cause issues if used as a flux in casting.

Finally, IFI member BHGiant recently shared a photo of an unsuccessful attempt to produce anhydrous borax in the oven. While this is a dramatic example of the expansion that can result just from the evaporation of surface water, it's noteworthy that the borax hasn't even come close to the 1770ºF/966ºC melting point. Indeed, I know of no commercially available household oven that can reach that temperature. 

20200726_214103.jpg.6884ec9977a9a7958ed9decacdc194a2.jpg

References:

The following quotes are provided for external confirmation of the information given here. The pages linked also have some additional information about technical details and other commercial uses, which some may find interesting.

From the Digital Fire Reference Library article on Anhydrous Borax:

Quote

Anhydrous borax is made by fusing hydrated borax into a glass and regrinding it. It thus contains little or no water of crystallization. The powder does not rehydrate under normal dry storage conditions. It is somewhat water soluble, but considerably less so than raw borax (in aqueous solution it can thus provide slow release of boron).

This material does not puff or swell during melting (minimizing loss of powder in kilns with strong drafts), and melts easier (the swelling in other forms can create a porous state with an insulation factor that slows melting).

From the Chemical Book website, a description of how anhydrous borax is produced; note that phosphorus pentoxide is a powerful dehydrator.

Quote

Most of the water of hydration is removed from the decahydrate (see below) by evacuation at 25ºC for three days, followed by heating to 100ºC and evacuation with a high-speed diffusion pump. The dried sample is then heated gradually to fusion (above 966ºC), allowed to cool gradually to 200ºC, then transferred to a desiccator containing P2O5 [Grenier & Westrum J Am Chem Soc 78 6226 1956]. [Becher in Handbook of Preparative Inorganic Chemistry (Ed. Brauer) Academic Press Vol I pp 794-795 1963.]

Thanks for reading; I hope this helps.

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  • Mod34 pinned this topic
  • 1 month later...

Thank you for the information being a beginner "almost I haven't actually started yet still building my forge" so I am reading a lot . This definitely helps me to understand how things work . 

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I'm IN! I love science, over my head or not. I don't have to understand or have much of a clue to read or listen to science stuff. 

For example. https://www.youtube.com/watch?v=eFIk7wpU5vA

I have no idea what she's talking about but she's a delight to watch and listen to. Well, okay I have a slight inkling but that's about it for my qbit savvy. 

Just remember a spinning head is a good thing, it keeps you stable. :)

Frosty The Lucky.

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1 hour ago, Frosty said:

Unfortunately for me, that sounds quite similar to this:

https://www.youtube.com/watch?v=Ac7G7xOG2Ag

Although I suspect every word she's saying in that video is true and accurate, my understanding of that topic is so limited that she may as well be pulling the same stunt as in the video I linked to.

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I'll be she told me what my problem has been all my life " not enough cubits" ! If I had know that I would have gotten some !:( Your right though I can listen to Dr Michio Kaku all day . He has a way of telling it that I can ALMOST understand !  lol . I think I will stick with history , that I understand completely especially our civil war !

Ok I forgot we are world wide here so let me correct " the American Civil war " but I love all history .

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I've recently learned that Trivex lenses are the best for both safety and prescription glasses. Just had my annual eye exam and my prescription has changed just a little. I plan on having my new glasses with Trivex which was developed for the astronauts face shields and is way better than poly carbonate.

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3 hours ago, Buzzkill said:

Unfortunately for me, that sounds quite similar to this:

https://www.youtube.com/watch?v=Ac7G7xOG2Ag

I LOVE that guy Buzz! I think he has a couple more, I have a friend who's a real fan, I'll ask him. 

Ahh Dennis. I listened to her for several minutes wondering what a biblical measuring system had to do with quantum computing. I finally realized I was Noah doubt wrong. I still don't know how many a qbit is. 

Trivex is better eh Randy? Cool as soon as my vaccine is fully in effect I'm up for an eye exam and new gasses. I'm over due, these are getting a little fogged up worn. I'm REALLY careful cleaning them but dust will still scratch poly carb lenses. 

Frosty The Lucky.

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  • 2 months later...

Dooodz - the Turbo Encabulator! HA-HAAAA! Haven't seen that one in years. I mostly agreed with what was stated, with the exception of the values he stated for the logarithmic bypass quotient. Clearly, his calculations failed to account for the encabulatory fibrillation caused by the fluxion generated when chrome plated knippling pins are incorporated into the design. A few tweaks to the inmolation intake settings on your defrundus valve will correct that.  ;-)

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Posted (edited)

Having read through, once, all text on this thread and yet, not up to speed on the back and forth of retro-encabulation, I arrogantly submit to the Forum:

that I am a  Dzhanibekov pseudoform wingnut with an idiopathic flip cycle mean of 67 seconds.  However like pi, it is, so far, a non-repeating value (thus, the idiopathy).

On 2/17/2021 at 5:52 PM, Frosty said:

I'm up for an eye exam and new gasses.

I renew my gases every 24 hours when under consitent pro-encabulatolion parameters...

Sci Ănce, Jerry?

JohnHCC, I am a fan of the Pentahydrate, as it is easy to fluff, smash down, and apply, and it has a satisfactory tendency to stay put...

Robert Taylor

Edited by Anachronist58
minutae addendum
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Basically, what you're doing is making anhydrous in the forge. Fluffing is the crystal water boiling out, smashing is equivalent to the grinding step, and it staying put on application is what anhydrous does: it melts where it's heated.

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Yes, quite.  I, being somewhat quarky (sic), find myself flustered by the fleeting fluffing in the forge as the flux flings off the first five waters. The final fives flight displays a more dignified egress.  It is possible that I have spent too much time staring into the flames...

Thanks, JHCC, for assembling a fabulous, and now, pinned, reference piece.

Robert Taylor

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