From: Rod Fleming (rodfleming@sol.co.uk)
Date: 05/05/00-02:21:56 PM Z
Hi
----- Original Message -----
From: Wayde Allen <wallen@boulder.nist.gov>
To: <alt-photo-process-l@skyway.usask.ca>
Sent: 05 May 2000 18:17
Subject: Re: Gum prints by enlargement
> I've often thought about modifying a welder's carbon arc as you suggest.
> There are a couple messy problems though. The worst one being that these
> generate a fair amount of smoke as the carbon rods burn up.
Most of this smoke is from the rod coating, called flux. It is there to
exclude oxygen, which would otherwise cause problems - basically the weld
would rust out from inside while it was being made. MIG &TIG welds, which
do not use coated rods, do not display this effect, (at least not to the
same degree) nor does carbon arc.
I'd guess
> that this is mostly CO2 and CO, but the welders rods are also usually
> coated with what looks like a copper coating, so that would be in the
> smoke too.
The "copper" rods are used in gas welding, not arc- it's just an anti-rust
plating. The balance of the oxygen/acetylene mix is set such that the flame
is essentially reducing, so oxidation of the molten metal is precluded.
Somewhere I have chapter and verse on what the flux coating of an arc
welding rod is actually producing in the way of shielding gas and I'll dig
it out if you want.
>You'd need a good ventilation system to handle these gases.
Yup. That's one reason why the lightsource would have to be mounted
remotely- consider a horizontal enlarger with a fancooled lightsource
outside the wall, connected to the neg holder by a bellows or a sliding
telescope tube for example.
> Another problem is that you need to continuously feed the rods into the
> gap as they burn up. That means you either need to control the gap
> spacing dynamically by hand or setup the mechanics to handle this for you.
Not that difficult. They used clockwork mechanisms for WW2 searchlights,
which were carbon arc. Personal experience with the carbon-arc brazing
attachments for hobby welders suggests that you'll get 5-10 minutes out of a
fixed position arc before the rods shorten so much that the arc dies. But
you are getting an awful lot of light in that time. A problem might be that
the intensity of the light from the arc may vary depending on the gap- I
have not tested for this.
> Finally, the carbon arc also generates a lot of heat, but then again you
> noted that too. It is used as a heating arc as opposed to a welding arc,
> and this heat will need to be dissipated.
Again, carbon arc searchlights have been around since WW1 when they were
used to seek out Zeppelins over the UK. This is not that difficult to do,
but it will be BIG.
>
> Yes, I think it should work. However, it does pose some interesting
> engineering problems. I haven't looked at any of the old commercial
> carbon arc plate burners, but I think if you are serious about this it
> would be a good idea. After all, there is no particularly good reason to
> reinvent the wheel.
You don't need to. Any hobby welding shop will sell a little low amp welder
of about 120 to 150 amps, with a "brazing attachment" (they are useless for
brazing BTW :-)for under £100. A quick check suggests that what you call a
"plate burner" is more or less the same thing as that which I'd call a
"brazing attachment", without the oxygen flow.
As I said before, I am not certain I know why you would want to go to these
lengths to replace the sun; but if you did want to go that far, I am
reasonably confident that carbon-arc would be a better solution than strobe
flash, which was where this began. You would need to design out the heat
issue and also the issue of deposition, because there will be some, even
though no flux is present, but these problems should not be insurmountable,
and you would have the huge advantage of not having to deal with potentially
lethal power caps.
Rod
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