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What does flux do exactly and can you weld without?

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Fluxes scavenge oxides from the surfaces, allowing you to get clean metal-to-metal contact necessary for a weld. Can anybody weld easily without it? Yeah, experts, in coal forges. Noobs in gas forges: lotsa luck! The rest of us, somewhere it the middle.

Original wrought iron welds to itself fairly well because it has low carbon content and silica strands internally, making it self-fluxing. Modern steels have none of the advantages, and the more alloying elements added, the harder to weld. Stainless steels require a really aggressive flux that is actually a breathing hazmat item when heated. 

In a *correctly shaped* joint, using the *right amount* of force, at the *right spots*, the hot flux squirts out when force is applied, taking the surface oxides with it. And the metal plates are joined cleanly, and without trapped flux or un-fused spots. Piece of cake baked alaska with a handmade fondant and piping roses. 

And yes, screaming hot flux spray means that you better have: visitor standoffs and screens, safety glasses, leather apron, buckets of water or a hose handy. You CAN set the grass on fire 30' away. Ask me how I know.

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Appreciate the responses! And yeah I managed to find some threads on using sand and little on why flux is used In general. So from what I’ve gathered so far your basically just trying to keep oxygen out of the weld along with any contaminates on the surface. 

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Flux does a number of things: I it shields the surface from Oxygen, 2 it helps "eat" scale/oxides already present and liquifies it so it can be pushed out of the weld, 3 it lowers the welding temp of the steel surface slightly.

Real wrought iron can be welded at temps above the melting point of the scale and with it's "self fluxing" make up can make astonishing welds.  If they wanted more they would use clean quartz sand or even ground glass---note any left on the work piece is quite difficult to remove!  Japanese sword and tool makers used a siliceous ash from things like rice straw and clay and again were welding at HIGH temps.

Modern steels, especially high alloy ones,  are more prone to damage at high temps and so are trickier to weld.  The use of Borax fluxes tracks quite well with the rise of Bessemer/Kelly steels and the other "modern" steels.  As I recall "Practical Blacksmithing" Richardson, 1889,1890,1891; discusses the use of Borax in welding of the "new" steels.

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Note to file.

Quartz is a crystalline form of silicon dioxide. (SiO2).

It is chemically the same as silica sand.

Folks,  please be careful with SiO2 inhaling a lot of it brings on a serious disease called silicosis.


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Hmm, interesting that it raises the boiling point as well. Hadn’t given that much thought.

appreciate the heads up slag.

Where would be the best place to get the dehydrated borax? The anyhydrous or how ever it’s pronounced? 
Can you get away with just the plain old 20 mule team or is it worth the extra money to get the dehydrated stuff?

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You can make it from 20 mule team, heat it and grind it.  Some ceramic supply places carry it. 

I generally just use 20 MT; but collect all the stuff that popcorns off and put it back in the barrel so it gradually gets more anhydrous as I go until I make up a new batch of 20 MT and RP (boric acid).

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There's a big difference between dehydrated borax and anhydrous borax. Let me explain:

Water is present in regular borax in two forms: moisture that's held between the particles of borax, and water molecules trapped within the borax crystals themselves (known as "crystal water"). Either one will cause bubbling as it turns to steam, which makes it harder for the borax to flow out on the surfaces to be welded, but it's actually the crystal water that's the big problem.

For the moisture between the particles, think about sand: if sand is completely dry, it barely holds a footprint. If it's a little damp, you can write your name in it with your finger. If it's very damp, you can make a ball and throw it at your sister. If it's too wet, it just flows away like mud. Similarly, if there's too much water between the particles, the borax will clump and stick together. If it flows freely, the particles are pretty dry: it's dehydrated. Baking it in the oven might dry out the surfaces of the particles (especially valuable if the box has absorbed a lot of humidity from the air and turned into a solid lump), but it will reabsorb atmospheric moisture until it reaches equilibrium with its surroundings.

The process shown in Jennifer's video is different because you are actually *melting* the borax, which destroys the crystalline structure and allows the water to escape. When the melted borax cools into a glass-like substance, there's basically no crystal water left inside: it's now anhydrous. When you sprinkle this on hot steel, it remelts without bubbling, as there is no crystal water left to turn to steam (there may be some moisture on the surface of the particles, but it's pretty inconsequential).

The "popcorn" that ThomasPowers mentions above is a combination of melted and unmelted particles, and therefore is a mixture of anhydrous and regular borax. It will therefore work better as flux than regular borax, but not as well as anhydrous.

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Appreciate the scoop jhcc! Good to know. So essentially you could buy the cheap 20 mt and melt it down to get the better flux without the trapped watering in it?

I’ll have to check that video out iron dragon. I’m at work at the moment.

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Nice info guys..  Thomas's 3 major points are  the main ones and the ones I stand behind..

Keep in mind Borax at elevated temperatures will Oxidize..    it will shield the metal but the flux itself will get all crusty..   For a nice clean weld the flux should be clear and runny.. Not crusty.. sometimes the crust is on the outside but be sure it is not crusty between the metal you are trying to weld.. 

The mole is the unit of amount in chemistry. ... A mole of a substance is defined as: The mass of substance containing the same number of fundamental units as there are atoms in exactly 12.000 g of 12C. Fundamental units may be atoms, molecules, or formula units, depending on the substance concerned.

Industrial Borax carried at my local chemical supply comes in 2 grades.. 

JHCC talked about the water trapped..    This is referred to as "Moles of water"..    They are 5 moles and 10 moles of water or  penta hydrate  and  deca hydrate.  

Either of these will work..  20Mule team is finely ground.. I don't know which it is Mole wise, but it seems to bubble less than the industrial 5mole or Penta hydrate Borax..  The granular size for the industrial stuff is fairly large compared to 20mule team. 

I prefer the boiled method which is shown in the video.. 

it takes about 4X the amount of volume of the white stuff  to make 1 volume of the boiled flux..    I like it better because of the increase in density..  I can use less of the boiled flux because it is a condensed product..     it will sometimes go 2 welding heats before I need to reapply it.   Because it is a more condensed product it covers better and faster and for some reason it melts are a little lower temp.. 

One other facet that happens is the flux eats the pan it is heated in..  So the addition of iron to the flux may or may not have something to do with it. 

I use which ever flux I have but only buy the industrial borax from the industrial chemical supply..    

As and FYI.. the company I bought my borax from (which I bought 2 80bls bags about 2 months ago) won't carry Anhydrous borax..    The guy said because it has no moisture when made it has to come in an air tight container..   He has ordered it before and he has gotten 80lbs of rock..  LOL. 

I personally have tried the oven baking method borax and for the time the boiled stuff I like better. 

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20 mule team works perfectly well and is inexpensive.


An 80lb bag bought in 1987 I still have 10lbs left.

I bought so much for teaching facility supply's. 

The argument agaisnt not boiling it can be legitimate.  Vit does boil the water out when applied.  

So much of it has to do with preference.

@JHCC did you see any difference?

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There are days when I sneeze and found I have accidentally welded stuff together in the forge and days when I could be working in a vacuum at 2300 degF and using polished pieces and still have problems.  When I have problems; I go back to first principles: Cleanliness, flux at low temps when starting, firm not sharp blows and I bring the heat up higher.  Generally it just  gives up and welds rather than arguing...

With billets I like to bring up the coal forge HOT and then turn off the air and stick the billet in to warm up in a totally reducing atmosphere and come up to a low fluxing temp.  Once the flux is protecting I run the air again till it comes up to welding temp.

Note if the flux starts getting crusty, it's usually an indication that I have a sizable clinker down in the bottom and it's shunting all the O2 over my work.  Clean the billet, flux while still hot and clean the fire and put the billet back.

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Yes that is funny isn't it..  Some days and some times the welding is like there was a magnet between the 2 and you just look at it and there it is.. 

I hate it when I pull the piece doing the drop method and I accidentally touch them together when not in the right position and that is it.. Argh.. 

I the thomahawk video that A36 didn't like any heats I threw at it..  I got it but it was fighting the whole time and I though the weld was super clean and solid and it popped open like a water balloon. 

Right on Thomas, Right on..  


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2 hours ago, jlpservicesinc said:

JHCC, did you see any difference?

I don't see as much difference in the result, but there is a difference in the process. That is to say, there's no substantial difference in the finished weld, but it's easier to get there.

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  • 1 month later...

Aren't most commercial welding fluxes anhydrous borax and iron filings?  I use twenty mule a lot, sometimes it seems to seep from the weld long after the initial weld is done if it's not dried first.  Another traditional one I used with success was dirt dauber nests, although I imagine they work better in sandy areas...

When I was casting a lot, I tried ground shell (originally to provide calcium to chickens) and even charcoal, but I don't think they'd work so well in forging, never tried them.

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The molecularly bound "crystal" moisture is known as "Hygroscopic Moisture" in the soils lab. A long slow baking at 230f or a little better will drive it off leaving anhydrous borax. You still have to crush it but it's not melted and glassy so it's a LITTLE easier to crush up. 

I buy Peterson's #2, blue a commercial welding, brazing, soldering, etc. flux at the local welding supply. It's anhydrous borax, boric acid and something proprietary making it blue. No iron filings or oxide. Peterson's #1 contains iron oxide as well. When I picked up my current can, most forge welding fluxes were running around $50. to $79. per lb. can plus shipping. The Peterson's was $26. per lb. on the shelf, shipping included and it works a treat. 

I run a propane forge and am usually successful but sometimes I might as well just go watch TV instead.

Frosty The Lucky.

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On 3/14/2020 at 1:24 AM, Nobody Special said:


Well, it will absorb water but i have found it takes a long time depending on how you process the Borax to begin with..  If you buy it all ready ground up it's pretty fine. 

the whole idea is to get rid of the puffing up for most. 

there is a whole bunch of information on the boiled Borax thread started awhile ago.. Nearly all the quesitons here were explained.. Sorry I don't have a link. 

My ability to edit the post is disabled so I can't change anything. 

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