[TowerTalk] Cadweld failing to ignite

[David Gilbert]

Neither a propane torch nor a MAPP torch is likely to be effective 
setting off the Cadweld powder no matter how hot you get it. Cadweld 
powder burns with an oxidation reaction, and the atmosphere in the 
vicinity of a propane/MAPP flame is primarily a reduction environment 
due to incompletely burned gas.  It's the same reason that the inside of 
a torch flame will remove the oxidation from a piece of copper sheet or 
plumbing fitting.   This topic comes up here from time to time, and 
almost nobody is successful using a propane torch with Cadweld powder.  
Match heads are a different story, though, and I've used them myself in 
a pinch.  They aren't completely reliable, but I don't see how they are 
any less safe than the normal starting material.

I don't use One-Shots since I made my own regular mold from a graphite 
plate and use standard powder that I bought on eBay.  I always saved a 
bit of the starting powder from every shot I made so that I could have 
some extra available if one of the shots was particularly balky.

By the way, I also played around with making my own Cadweld powder from 
scratch.  The reaction is a pretty simple one involving only copper 
oxide and aluminum powder*, but I learned that the particle size of the 
ingredients is critical for having a controlled reaction.  Coarse 
particles are very difficult to fire off, while the fine stuff 
(laboratory grade) I was able to buy goes up like the flash powder 
photographers used to use.  If you compare the Cadweld starting material 
with the normal powder, that's pretty much what you see ... the starting 
material is simply finer grain.  I suppose if you were willing to risk 
losing an eye or having your hand blown off you could try grinding the 
regular powder into finer particles, but I would strongly recommend 
against it.

The best advice I've seen here so far is from the guy who recommended 
you just throw it away and use a new one.

73,
Dave   AB7E

* 3CuO + 2AL=Al2O3 + 3Cu + HEAT (lots of it) + LIGHT (lots of it)




On 1/21/2013 2:53 PM, K8RI wrote:
> On 1/21/2013 4:23 PM, SPWoo wrote:
>> Hi All,
>>
>> I'm having a problem where one of the cadweld one-shot doesn't have 
>> any of the igniting powder in it.  From what I've gathered the main 
>> powder material needs a very high temperature to ignite.  So far I've 
>> tried a torch (MAPP gas) but still the main powder fails to ignite 
>> despite the powder was glowing red. Someone has suggested using a 
>> magnesium fire starter.  I didn't have any problems lighting the 
>> other cadwelds with a flint but what does one do if there's no 
>> igniting powder?  Your help is greatly appreciated.  73.
>>
>
> Just a propane torch should ignite them.  I've ignited them with match 
> heads from the old kitchen matches, which isn't very safe and I do not 
> recommend.
>
> If it was glowing, it should have ignited.
>
> We're missing something.
>
>
> 73
>
> Roger (K8RI)
>
>
>>
>> Best Regards,
>> Jonathan Woo, W6GX
>> (970) 646-1711
>> _______________________________________________
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