Thermit powder in UK

Get yourself a file and some metal and go for it!

Buy the DVD from Ribbands Explosives and enjoy. Far safer IMO.

Chris

Hadn't considered coal mines, though same thing should apply. Rules for specific cameras may relate to their electrical gubbins - and lighting (flash bang wallop - kaboom).

I assumed the reason we don't get much film from coal mines was the predominance of black. Not that that would stop David Attenborough.

Steve

Nice to see so many young people and read their comments.

The answer for us older-sorry, ancient ones- is the incendiary bomb which was no more than an intimate mixture of iron and mgnesium filings/turnings. Classically, something like a ribbon of magnesium is required to ignite the lot. Once it fires there is precious little conventional way that you can put the garden hose on to extinguish it. It simply explodes again!

So, how does one extinguish the beast? Some daft ossikins will have a go at playing Herman Goering and re-enacting the Blitz. There are two ways, the first is to put a sod on the top( not you Momma, sit down) or use a fine sprinkler of water which will cool the reaction down slowly.

Following on, the industrial use was to join tramlines! Never more serious. The end product is a lump of pure-ish cast iron.

Mind how you go!

Oh, and Steve? Sorry, but you start at Davy Lamps and not Davy Attenborough!

Norman

The electrical aspect of batteries, switches and motors was the obvious cause for concern and clockwork cameras were available to use if this was a possible problem. But the last time I put cameras down a mine (in the Rhonda - miners' strike) the main concern was aluminium

- presumably mine safety officers were happy about the safety of the electrical aspects of the equipment

Lots of factors used to affect it - including lack of lighting, lack of suitable cameras for sound shooting (clockwork) and lack of good sound gear (electrical considerations). The BBC actually had a clockwork powered tape recorder to use down mines with suitable protected electronic circuitry, but it was not easy to use.

The programme shot in the Rhonda - about the miners' strike and its aftermath - was the first where we were able to put a 'proper' sound camera down the mine along with standard sound gear, and the advent of high speed film stocks allowed the used of available lighting to get good or acceptable results, with a lot of sync shooting at the coal face. The programme was "The Last Pit in the Rhonda".

I suspect that a reason that you don't see much film from coal pits these days is that there are not many of them left!

Jim.

Jim,

I'm going off the thread here, and I don't think we even started near model engineering - but you aren't by any chance a black belt on sound recording ? I have been wondering how I could capture the sound of a dragster at Santa Pod. Modern electronics can store a digital signal, but I don't know what sort of microphone would handle the VERY high sound pressure levels. Any ideas (apart from standing 500 yards away) ?

If you know of a better newsgroup to pitch this one, then please advise.

Steve

Steve,

It's years since I was 'on the road' but for situations similar to what you state, a good dynamic cardioid mic would do the job. I used to use a Sennheiser MZH214 (I think that was the number) which would stand up to anything - even pop group mega watt rigs at full blast. But that was thirty years ago and I've no doubt that the mic has long been obsolete. But there should be modern equivalents.

The thing you have to watch out for when recording into digital is that you can get a lot of transient distortion caused by high transient peaks in loud, edgy signals. In the 'old' days of analoguie recording, a good analogue recorder would gently roll off on these transients and give good results. But digital recorders wont accept these high peaks happily and will go into distortion.

Since the signal to noise ratio in digital recording is very good, it is usually better to record signals with high transient content at a low level so that the transient peaks don't drive the original recording into distortion, then bring the levels up later when you can control the transients using filtration of some sort - e.g. a low pass filter to limit the high frequency content of transient spikes.

Jim.

Thanks, Jim

I will take heed of what you say. I hadn't thought about pop gigs as I always expected they would be recorded off the mixng desk and a bit of audience added later.

Steve

Steve,

Not if you're filming reality of a pop gig :-) I used to have a set of headphones which were ear defenders with small AKG headphones inserted - just to protect my own ears in the situations.

Live recordings of pop gigs were usually done multi-track with the mix downs done at a later date, so it was not easy to get a mono or stereo mix down on the night. You could sometimes get a mixed feed from the PA mix, but that could have problems as well since that mix was geared towards matching the hall acoustics and dynamics. What often happened was that you recorded actuality sound at the time and used it as a guide, then the pop producer would supply a mix down version at a later date which would be substituted in the programme sound.

Jim.

I agree with Jim but would suggest an omnidirectional mic. The cardiod will have a tendency to give a bit of a whooshing sound(phasing) as the cars move past. This will depend on where you're standing of course.

About 15 years ago I did sound for an NHRA National drag race. Standing between two top fuel dragsters as they launch is pretty unnerving the first time. The asphalt vibrates enough that it's hard to stand. The recommended mic was an Electrovoice 635, in the U.S. of course. If I recall correctly the 635 was calibrated at the factory at

150 dB SPL so it was certainly up to the task..

Oh yes, wear ear plugs. Don't worry about monitoring just watch the meter.

Kelley

Thanks for the (non hijacked) replies!! Can't really be bothered filing down large block of aluminium, although a quick glance under my car gives me no problem in finding the iron oxide...

Perhaps I'll apply for a lab technician's job in a school Chemistry lab...

In article , Robin writes

Saw all the other interesting replies. I had some experience of the thermit reaction in my teens. As others said, it is very hot but very, very difficult to start reliably. The way I did it was to use a small amount of barium peroxide as an accelerator, inserted in a depression in the thermit mixture, then embed the magnesium ribbon into this. That got fairly reliable starting.

Unfortunately, this is unlikely to be easy, since barium is a highly toxic heavy metal and BaO2 may even be a scheduled poison. There must be some other ways of doing it, I cannot imagine that early railwaymen went round with pockets full of the stuff.

What no-one has yet mentioned is that the thermit process also works very well with chromium (III) oxide (Cr2O3). Interestingly, this is the end-product of the decomposition of ammonium dichromate. The latter, an orange crystalline powder, when ignited gives of clouds of pungent gas and expands to many times its original volume of fluffy Cr2O3, just right for the thermit process. I think it used to be popular for making indoor fireworks.

Both the Fe2O3 and Cr2O3 versions produce molten metal (Fe and Cr respectively, obviously). Since the melting points of these metals are

1535 and 1857 deg C respectively, this gives some idea of the exothermicity of the reaction.

As someone said, aluminium filings can be made with nothing more sophisticated than, well, aluminium and a file. To get the most reliable starting you would need the filings to be quite fine though. Timstar (educational supplier, pdf catalogue available on the web) sell Al fine powder for £4.40 for 250g, though they may ask for a reason for purchasing ("I'm a bit of a pyromaniac" may be sub-optimal here). Bear in mind is that Al powder and ammonium nitrate makes a fine explosive (used IIRC in WW1) so be prepared for such questions.

Don't know about the legality of do-it-yourself pyromania. A few years ago I would have thought nothing of it, but in these anal times (does anyone think as I do that the cure may be worse than the problem?) I'm not sure; if you can be convicted of "having information likely to be of use to a terrorist" then any chemist or ex-chemist like me is automatically an outlaw!

I'm a bit sceptical of the aluminium in coal mines bit. Al does burn quite fiercely in air, but it is really hard to ignite as it is protected by a thin impervious film of Al2O3. Warship superstructures are still made of it, and (recalling the Falklands) when it does go it goes very well, but there it took Exocet missiles to ignite it. Also, most aeroplanes are made of Al alloys. I think its vulnerability to fires (its melting point is quite low) is a more likely explanation than spontaneous friction ignition in contact with rust; I have to say I have no firm knowledge and could be wrong, but if anyone has any authoritative references it would be interesting to see them.

David

At a guess, powdered Titanium would substitute well for the Al too.

Regards, Tony

Thanks for the reply, David. I'll see what they say - might have to pretend I'm an artist or something... I'll think up a convenient excuse when the time comes!!

David,

Interesting that it can be used for other metals, had never considered that, but then had not spotted a source for the oxide.

Agree with you about issues of purchase. My grandfather used to make his own fireworks, but I suspect that big brother will these days detect you have been purchasing the ingredients and next thing you know you are on the suspect list - and you may get a visit. I think many things are classed as 'notifiable', for one reason or another. My advice is to try to avoid any experiment that involves ingredients that could be mis-interpreted (and that is not easy).

About the aluminium in mines, it is not that it catches fire, just that if you bang it on a rusty surface you have for a fleeting moment the ingredients of the thermite reaction, and the knock both removes the protective oxide film and injects sufficient starting energy that you can get a mini thermite reaction - its only tiny and very brief, but if you have a flammable mixture it will cause an ignition - and kaboom - mining disaster !

Incidentally, another interesting exepriment we did at school was to put mercury on aluminium foil. It prevents the formation of the protective oxide film and it reacts spontaneously. If it wasn't for the oxide film, aluminium would be as dodgy as metallic sodium - that would cause aircraft designers a few headaches !

Steve

Sounds a bit dodgy trying to order some of this stuff, so I'll probably just drop it...

...anyone know where I can get some plastic explosives?

Hence the restrictions on carriage of thermometers in your aircraft luggage.

Russell

In article , Peter Fairbrother writes (Snip)

(Snip)

One of my friends gets to blow stuff up for a living.

And she gets *paid*.

I can't figure that one out at all.

Navy pilots get to fly, blow things up and get paid.

The 'fishheads' move the airfield about while they're doing this out o jealousy.

Robi

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