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Thursday, 16 October 2008

Post #90 A Science Experiment



I was in a store a few days ago and saw fire blankets for sale. They're non-flammable fabric squares which can be pulled from a package hanging on your wall and thrown over any fire involving the sort of fuels which don't respond well to application of water, e.g. cooking oil.

It triggered a memory from school days about the misguided efforts of a teacher to demonstrate the process known as "sublimation". This is the direct translation of a substance from a solid to a gaseous form. This could have been a difficult thing to demonstrate in the bare-boards science classroom that my high school featured; things that will turn from solid to gas at room temperature usually aren't lying around loose on shelves, for obvious reasons. However, an apparently fortuitous circumstance helped out the would-be demonstrator. In the classroom was a CO2 fire extinguisher. The enterprising teacher figured that he could show us the process by improvising with this device. He pulled the retainer pin from its trigger and blasted the extinguisher repeatedly against a blackboard. After several minutes of this a lump of solidified CO2 was sticking to the board. We were then exhorted to observe how it vaporised and disappeared into the air. For the benefit of those slow to catch on, or having trouble seeing from the back of the room, he put more of it in place with repeated furious blasts from the nozzle of the extinguisher. I didn't think this was all so amazing. Perhaps if a lump of solid metal had disappeared, it would have been worth the build-up, but this exercise just seemed to me to be showing the boring obvious and to be a waste of the contents of the extinguisher.

A week later when the great "sublimation" show was a fading memory, the same teacher was placing various items on the lab bench at the front of the room for an experiment when he spilled some kerosene on the bench top. He made a rapid grab at the flask he'd spilled it from and knocked over a Bunsen burner. The burner was operating and ignited the kerosene which was now spreading along the bench-top. The teacher thought he'd settle the burner down by pulling its supply hose from the gas tap at the end of the bench (The burner was lying in a spreading pool of burning kero by now). That stopped the burner fuelling the fire at its end of the hose but allowed gas to vent directly from the tap. The valve had a habit of sticking open and removing the hose plug hadn't popped the valve out to stop the gas flow. Of course the gas stream now ignited from the flames coming from the kero. It came out at full pressure, not the small flow the burner's outlet valve had been set to, and it was blazing a metre-long flame down the bench. One of my classmates was of Austrian ancestry and was a World War Two buff. He called out, delightedly, "Achtung! Flammenwaffe!!"

Now there's a dilemma: You've got burning kero spreading down the bench towards containers of volatile materials standing at its end (sulphur powder, acids, alcohol, other such goodies...). You've also got a gas flame a metre long that's doing God-knows-what to the gear on the bench top which is also surrounded by the burning kero. What do you do first, fight the kero fire or try to shut off the gas? I didn't know then and I still don't, but I guess there was a main tap near the bench or under it from which you could stop the gas. The teacher probably knew but he didn't seem to remember...anyway he decided that the kero was first priority. A reasonable decision; the gas flame was of finite length and coming from a fixed outlet, the kerosene was spreading fast along the four-metre long bench. He decided to give the kerosene the benefit of the trusty extinguisher's attention. In a smooth, commando-svelte motion he turned to the red cylinder of salvation resting in its wall bracket, hefted it, pulled the retaining pin (...close breech cover, draw back actuator lever...), pointed the uncompromising black metal nozzle at the kerosene fire and pulled the trigger lever. A blast of ice-cold fire-smothering CO2 jetted forth. For one second. Then, nowt but a feeble puff. Yes, he'd tested it to death. It was empty.

Now, you may be thinking, "What were the students doing while this..." Those of us at the front were taking a keen interest because the first students' bench was actually right up against the front of the lab bench. I was sitting at that bench with several others and was collecting my equipment and preparing to retreat from the kerosene which was burning only a few centimetres away. The others in the class were just watching, surprisingly, without laughing. There were about thirty of us in the room and no bastard, me included, was trying to help the guy. Anyway, there wasn't really a damn thing we could do. This is why the fire blankets in the shop triggered my memory of that day. They would have been just the thing to stop the progress of the fire. There weren't any in the lab. I don't believe there was one in the entire school.

Having discovered that his mate, the extinguisher, was extinct, the teacher cast about frantically for an alternative. Not having a suppressor blanket he applied his improvising talent again and seized upon a large cloth lying on a side table. Now that cloth had been hanging around the lab for years. It had done sterling service as a wiper-up of messes of all kinds and had collected within its fibres every chemical which had ever been used and spilled or dropped in that lab.

I once heard an American commentator on international affairs use a metaphor to describe errors in US foreign policy which was a story about a man walking through a perilous forest: "As he's walking along, in the gloomy light under the forest canopy, he sees a stick lying on the ground ahead him, a small curved branch that's fallen from a tree. In the poor light he mistakes the stick for a snake and panics. Casting about for something to hit the snake with, he sees something lying on the ground behind him which he wrongly believes is a stick...and he grabs it up..." That perfectly describes the situation of that fire-frightened teacher. That old cloth was probably the most flammable piece of textile product within the borders of the Commonwealth of Australia. And that's what he chose to beat the fire out with.

It caught in one millionth of a second and he was waving a blanket of fire as he spread the still-burning kerosene even further. I also remember that every speck of old spillage adhering to the bench-top was now igniting in this ideal fire environment. Little spurts of purple and yellow flame flickered into view for a fraction of a second as chemicals were liberated from the bench surface by the heat of the fire. Brilliant white sparkles showed where long-forgotten magnesium powder spills had occurred. At least it was getting a good clean-out.

As the cleaning-cloth was now beyond being held in a bare hand it was thrown to the floor and the teacher began futilely stamping on it. He had to give up and let it burn as the kero fire licked at the containers of combustibles standing at the end of the bench. He began grabbing them and transferring them to a waist-height shelf that was along the side of the room at right-angles to the lab bench. In this, at least, he succeeded.

Now he had a real inspiration. The lab bench had a metal sink. He decided to squeegee the burning kero into the sink using two large steel rulers that were on the blackboard shelf. He pushed the kero back, corralled between the rulers, and forced it over the rim of the sink. This required some suffering on his part because those rulers were excellent conductors of heat, but he'd beaten the fire at last. Most of the kero was in the sink, the little left on the bench was burning out. The burning cloth was now a smouldering, greasy black twist on the scorched wooden floor. That just left the flame-thrower gas tap. Picking up a piece of steel tube from under the bench, he advanced on the outlet, reached out and tapped it with the pipe. The valve popped out and shut off the gas.

There were some moments of silence as we all surveyed the now-quiescent scene. Then a burst of applause and sarcastic cheering. "All right, settle down, you blokes", said the teacher and proceeded to use some unburnt cloths to mop up the mess he'd made.


I always laugh at the memory of that episode but I knew from the moment it happened that it was nearly a catastrophe. I'm ever more amazed as the years pass at how ill-judged he was and how ill-equipped that place was. The thing that beats me most is how a man past thirty could be so foolish as to waste the contents of the fire extinguisher in the one room in the school where it was most likely to be needed. I have wondered if he had to tell the headmaster that he'd used up the CO2 in a dopey experiment. It's occurred to me that he may have said that he used it up in fighting the fire. Maybe he never said a word and just left the empty extinguisher for someone else to find out about the hard way. None of the students would have gone to talk about it with the headmaster. They were different days. There were lines you didn't cross. What happened at school stayed at school. What happened with a teacher might be discussed with other students but no-one would have crawled to the boss and informed about it. Today he'd be up to his neck in that burning kerosene.

2 comments:

Dina said...

Dangerous and scary....but probably most of the students learned more there than they did in any other science class!!!

Retarius said...

The funny thing is, I can't even remember now what the original purpose of the experiment was.