Saw dust Flux: Why not use this instead?

[milprileb]

Gentlemen,

The use of sawdust as a flux as detailed many times on this forum is very effective. Its a great flux. However, it gets smokey.

I was wondering why charcoal could not be crushed and used as a flux. No smoke and basically the same results. Its cheap enuff !

[btroj]

That smoke is important. It means that the top of the melt is held in an oxygen poor region. This is part of what allows the flux to reduce the oxides back into the melt.
Others will chime in with other advantages but flux is like a lube, it plays many, many roles. A substance may do very well in one role yet fail in others.

[2wheelDuke]

I'm pretty sure others have said that charcoal does just fine as a flux. I think the main difference is that the sawdust is a byproduct that's obtained for free.

[geargnasher]

Charcoal, even "activated" or porous charcoal, isn't very effective for reducing oxides. It's ok for "fluxing" or soaking up the elements we consider impurities to our alloy without removing the good stuff like tin, arsenic, copper, and antimony, but it doesn't do a very fast or good job of reducing oxide scum, here's why:

The reduction of oxides, and by reduction I mean initiating the the chemical process called reduction/oxidation reaction, is accomplished by actively oxidizing (in this case, combustion) one element and robbing outer valence electrons from another element which has already ben "oxidized". Basically to get metal oxides to "reduce" to their elemental state, something else has to be oxidized. In the case of grease/oil/wax/rosin/sawdust it's hydrocarbons, not inert carbon, that gets burned or oxidized, and that process of gaining electrons takes them away from the metalsl, which is what we want. This has to be an active process to be effective. That means an active smolder or flame. Light the smoke if you can because that helps create a low-oxygen enviroment at the melt's surface, and the poor combustion taking place makes lots of carbon monoxide, which is very reactive and wants to become carbon DIoxide, so it takes oxygen from the oxidized metals in the pot.

A layer of smoldering sawdust, particularly if it contains a lot of pine sap, is the most effective reducant and flux anyone has been able to come up with. It does three things:

1. Sawdust burns slowly and inefficiently, so it creates a carbon monoxide/dioxide shield at the melt's surface, which both excludes oxygen from the surrouding air and sucks oxygen from the metals, thus reducing them.

2. Any sap present will donate additional reactive hydrocarbons to the process, speeding up and fueling the redox reaction. A blob of boolit lube, paraffin, beeswax, etc. will also do this.

3. Sawdust is a highly effective FLUX as Glen Fryxell has discovered, which cleans the melt my absorbing certain impurities like a sponge. Impurities like aluminum, calcium, iron oxide dust etc. are removed if the pot is stirred well with the sawdust to maximize the melt's exposure to it.

So, you can see why plain carbon isn't all that great. It will serve to help keep the surface from re-oxidizing if added after the melt has already been reduced/fluxed/skimmed, but it is "dead" or inert, so it has limited reactivity to metal oxides.

Gear

[skeet1]

To reduce the amount of smoke get a butane lighter with an extension on it, the kind you use to lite a grill or your fireplace and when the sawdust starts to smoke just ignite it. The flame will creep over the entire layer of sawdust and burn it. This will reduce most of smoke, it will not eliminate all of it.

Ken

[geargnasher]

Let me pose a question: If your setup can't exhaust a little smoke from fluxing, then how will it eliminate the airborn heavy-metal dust/vapors from your breathing air?

Gear

[birdadly]

Can I be selfish and ask a question here for clarity... here's my process from my first batch in my LEE 4-20:

-get lead melted
-put teaspoon of sawdust on top (it starts to smoke)
-ignite the smoke

When exactly should I stir the sawdust into the lead?
And I think I read somewhere that, if using a bottom pour pot, to leave some (or all?) of the dross on top while bottom pouring?

gear-- if using a sawdust w/out sap, would you suggest adding a small chuck of wax as well? I'm using sawdust from a hand sander, that's not too fine is it? Thanks! -Brad

[oneokie]

Can I be selfish and ask a question here for clarity... here's my process from my first batch in my LEE 4-20:

-get lead melted
-put teaspoon of sawdust on top (it starts to smoke)
Personally, I would use more, like a handfull-
ignite the smoke
I never light the smoke from sawdust, however I do light off the smoke from wax
When exactly should I stir the sawdust into the lead?
As soon as you add the sawdust

And I think I read somewhere that, if using a bottom pour pot, to leave some (or all?) of the dross on top while bottom pouring?
Correct
gear-- if using a sawdust w/out sap, would you suggest adding a small chuck of wax as well? I'm using sawdust from a hand sander, that's not too fine is it? Thanks! -Brad

Hope my thoughts help you.

[geargnasher]

Oxide scum contains a large percentage of tin and antimony due to different rates of oxidation compared to lead, so if you want your alloy to be consistent, keep the "dross" or oxide layer reduced back into the melt. If you leave a layer of sawdust on top of your casting pot, it will seal the surface, reduce oxides, and keep working for you the whole casting session unless, of course, you are ladle-casting.

Gear

[birdadly]

Oh you two and your answers are awesome, thanks so much for breaking it down one more time for a newb I hope it helps at least one more guy like me that just didn't quite know the exact ins and outs! -Brad

[noylj]

Heavy metal vapor? Where?
Heavy metal dust? Keep area clean and vent out. Don't breathe any of the dust from the dross.
I'm still old school, so I start stirring the pot as soon as the flux smokes/burns. I still flux vigorously and often.
I keep a layer of vermiculite over the melt--had a moth once come for a visit and the I got lots of tinsel around the work area.

[geargnasher]

Noylj, go get some lead indicator wipes and go over your casting area. How did it get on the walls? On your vent fan? Does it matter? It's airborne, doesn't matter what form it takes, you can breathe it without adequate ventilation. How a moth caused a pot explosion is beyond my ability to comprehend unless it was stuck to the bottom of an ingot you added.

Gear

[MikeS]

I don't have any sawdust, but I do have a package of pine shavings, the stuff used as bedding for hamsters, will that work as a flux as well as sawdust, or at least close to it?

[kbstenberg]

Hey Mike S You do know you have to use the shavings before the hamster does!!

[Charlie Two Tracks]

I tried the charcoal. I crushed a briquet up and put it on top of the mix. Stirred it in and it was pretty dusty. It turned into a very fine dust on top of the mix. It didn't hurt anything but I live near a lumber yard and get all the sawdust I need.

[smokemjoe]

I have used wood ashs after they have been shifted well, Any pieces of charcoal will burn again. Works good. Joe

[MikeS]

Hey Mike S You do know you have to use the shavings before the hamster does!!

Well, you could use them afterward, as long as you have no sense of smell Besides, the closest thing to a hamster around here are some Min Pin puppies, but I'm keeping them away from the hamster bedding!

[geargnasher]

I dunno, dog pee on used wheel weights seems to help the fluxing process.......

Gear

[Boolseye]

Gear,
I'm glad you understand the chemistry; It's cool to understand why it works so well.
(I flunked chemistry the first time, had to take it again. If we'd done reloading and casting I bet I would have done better!)
-jp

[Boolseye]

-get lead melted
-put teaspoon of sawdust on top (it starts to smoke)
-ignite the smoke

When exactly should I stir the sawdust into the lead?
And I think I read somewhere that, if using a bottom pour pot, to leave some (or all?) of the dross on top while bottom pouring?

A couple tablespoons of sawdust, stir that in...lotsa smoke...keep that stirrer stick around for the whole session, you can use it to clear a place at the top of your melt, say to heat up a lee mold, then cover it back up with the charred sawdust. The stick does some fluxing, too. Yep, you rite, leave the dross on top for the majority of the session

-jp