A definite fire hazard

When I joined the BBC as a Technical Operator in 1961 we were still doing a substantial amount of recording on direct-cut disks, usually known as acetates. In fact they had an aluminium base and a coating of cellulose nitrate in which the groove was engraved: most shorter items were recorded this way rather than on tape, at 78 rpm - they could be played back in studios (which had no tape machines in them then). The quality was surprisingly good - in fact better than 15 inches-per-second tape on the first play, though they wore rapidly as the surface was much less hard than a commercial pressed 78.

When you cut a disk the off-cut comes off as a ribbon, called swarf. Although the cellulose nitrate is safe enough when it's on the base, because this swarf is mixed with air it is extremely inflammable. When I was on the training course at Evesham the lecturer took a handful of the stuff outside onto the concrete and dropped a match on it (I suspect this was to discourage us from trying it). It went up immediately in six feet of flame, and I could feel the heat on my face ten feet away - it's that dangerous.

There is a legend that, at the BBC Studios in Oxford Street (which closed in 1957) a producer (later a well-known politician) knocked his pipe out in a bin marked 'fire' (thinking it contained sand) and the swarf in it set fire to the recording channel. At Bush House another producer dropped a match (which had gone out but was still warm) in a bin - this time the engineer had the presence of mind to put his foot on the bin, so there was no fire, but the fumes got into the ventilation system and caused the evacuation of the entire floor.

One of the cleaners saw some of this soft black stuff and took it home and stuffed a cushion with it... very fortunately someone found out about it and she was discouraged. She might as well have taken an incendiary bomb home - if someone had ignited that it would have burned her house down.

Fortunately digital systems don't present these hazards.

Click here to see a large version of the photo.

Posted: Tue - February 6, 2007 at 09:24 AM by Roger Wilmut          



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Published On: Mar 11, 2016 05:00 PM