PhotoHistory

From: THE BRITISH JOURNAL OF PHOTOGRAPHY
No. 1392. VOL. XXXIV. JANUARY 7, 1887

OXYGEN MAKING

By LEWIS WRIGHT

Perhaps a few hints upon the above subject may be of service at the present season, and may save one or the other reader from some trifle of expense, because when properly purified the gas may just as well be made from the common and cheap chlorate, which reduces the cost materially; and from trouble, because after some experience the time of the operation may be reduced a great deal.

Retorts

Cast-iron retorts are a great deal the most lasting, though I suppose if an explosion did occur their fracture would be more dangerous than that of wrought iron. They can be got made specially, which are better than the old mercury bottles, and more convenient; one such costs about 12s. 6d., and will last a lifetime. A friend of mine adopts another plan. He bought a Papin’s digester at Benetlink’s, and got them to fit a bent gun-barrel to it. Thus he had a retort with a safety valve which is very handy to wash out, the top coming quite off, and leaving a mere open pot to clean. The tube should be amply large to avoid choking with manganese dust, and I must say I much prefer to have a safety cork somewhere. It would not be much use in case of an explosive product, but it would prevent any bursting through a choked pipe, which has happened by carelessness, though it never ought to. No one with any experience buys a copper retort, which is only adapted to yield a few more shillings to the seller for no value whatever. The value of wrought iron depends upon the thickness, of course; it is bound to be eaten through in time by the nascent gas.

Making Gas

There is nothing equal to a gas furnace, of almost any sort; but it is better if possible to have the whole affair arranged so high from the ground that the flame can be seen and the tap adjusted instantly without uncomfortable stooping. The proportion of material stated in the manuals is one part of powdered manganese oxide to two parts of potass chlorate, by weight; and it is best to begin with that. Buy crystals of chlorate, not powder, but crush to small size all unusually large lumps, and pick out most carefully any tiny bits of straw, or paper, or wood, which in the cheap quality are sometimes found swept up from the floor of the store bin. All this can be done in a minute or two for a lot of fourteen pounds, spreading out the whole heap on a sheet of the Times. If not done the large lumps are generally wasted. The proportion named will hardly ever give any startling rush of gas unless a very fierce furnace is employed. Turn up the flame well till the early bubbles have come over, and then turn down a little, but not much. In nearly all cases the gas will come over steadily. Allow about one pound chlorate to four and a half feet of gas. The best plan is to mix a lot at once — say fourteen pounds of chlorate with the manganese, and then keep the mixture in a large tin canister with a hinged cover. It is another good plan to cut an old coffee canister to a size which just holds the “charge” necessary to fill the bag, as found by trial, and also to keep in the large canister a tin funnel with large spout, and a small scoop I use a garden trowel, but made with a back like a scoop. Then every time you can shovel up your exact charge, and funnel it into the retort at once. It should be gently shaken evenly over the retort floor, or roughly leveled, and not left in a heap.

But, after a little practice and watching of the behavior of the stuff, it is well gradually to reduce the proportion of manganese, first to one in three, and finally to one in four, or less. The gas will now come off much more quickly, so that a ten-feet bag (40 x 31 x 24) may be filled in fifteen minutes; but it would be by no means safe for a novice to begin with such a proportion. As soon as ever the bubbles come quickly, it is necessary to turn most gas furnaces down to about half flame; generally even so the gas soon increases, and, rather before it has got to the rush considered safe, the flame must be lowered still more, or something or other will be blown off by mere pressure. When in full swing the evolution of gas will go on with little check long after the flame is quite out, and it is this behavior which the beginner has to learn before he tries to hurry matters. On the other hand, at first he will turn down too much sometimes in over caution, and the gas will stop, when it takes a good while to start it again. Getting a bag filled quickly really depends upon avoiding both mistakes, and by timely adjustment keeping the process going at a fairly rapid rate from first to last. This rate is much faster than a novice would suppose at first, but it would not be safe to work up to it otherwise than gradually.

Such a rate of work will always deposit some manganese dust in the tube between the retort and first wash bottle, and it is very important that both the retort pipe and the rubber tube be perfectly cleared before the next operation. To avoid this, some use the manganese finely granulated instead of powdered. I have had no experience with it in this state. The well-known caution about any possible contamination with carbon need not too enlarged upon here; but we cannot be too particular in insisting, under all circumstances, upon the rubber tube being taken off the retort delivery tube before the retort is removed from the flame. If this is neglected, and the retort should become perceptibly cooled while still connected with a wash bottle, there will be a partial vacuum, water would be sucked back into a highly-heated iron vessel, and the result would be a steam explosion. It will be seen, therefore, that under given circumstances there might occur either an over-pressure explosion, a carbon explosion, or a steam explosion. To see these things is to disarm them. The arrangements immediately described offer an effectual safety valve against mere over-pressure. But the pipe, from retort right down into the wash bottle, should on no account be less than half an inch clear bore, rather more preferable.

Washing

A large proportion of the gas marie is never properly purified, and the consequence is a rapid destruction of bags and taps by no means necessary. If a tap shows any perceptible corrosion much less green rust about it the gas is used when foul with chlorine. The same if the gas has any pungent odor, from which even fairly pure oxygen is perfectly free. There is much popular error on this point.

1. Water alone will not purify gas;

2. One bottle will not do it unless nearly a yard deep.

As to the medium, many use either potassium carbonate or sodium carbonate (common washing soda), and either answer fairly well, but will not purify a very foul sample. I prefer to use caustic soda or potash, which will entirely remove the chlorine. Caustic soda is usually sold in sticks about five inches long: use a couple of sticks to a gallon jar. The same solution of this may be used three or four times over, which saves trouble. The oxygen should be passed twice through this, and may then be regarded as pure.

Common washing bottles with their screw caps are very troublesome. For years I used vulcanised india-rubber Woolf tops, with rubber nozzles through which the pipes were passed, stretched over the necks of the bottles. These are much better. But for a still better arrangement, combining the maximum of efficiency with the very minimum of trouble, I am recently indebted to Mr. Charles Darker, an ingenious artist, for he is truly such in all optical matters to whom we are all indebted for more than many are aware. His cap for washing bottles has the delivery pipe of brass going down into the solution. This passes through and is soldered into the cap, into the side of which is soldered the exit tube. And over the cap is stretched about two inches of thick iudiarubber tube. The whole is made so that the lower end of this stretches over the mouth of the usual size wine bottle. On a journey, therefore, these can be used, and no bottles need be taken, nothing but the caps and tubes. And whenever the rubber loses its stretch, there is only this piece to renew; the rest is as good as ever. It is put on and removed in an instant, and the whole arrangement speaks for itself. Over-pressure will force it off the bottle, and to this danger it therefore acts as a safety valve.

One of your contributors, I think it was Mr. Scott, was, however, quite right last year as to the desirability of large wash bottles, and for home use it is better to use gallon size, which is easily got of the same sized necks. Use two of these, nearly full, and the gas will be pure. It will, however, be also moist, which is bad, too. To avoid this, use a third cap on a narrow bottle, empty or dry, such as a thin champagne or bock bottle, or a piece of large glass tube. The moisture will collect in this and only dry gas go into the bag. The tubes should go nearly to the bottom of the wash bottles, and the side holes not be bored very high up, or the gas will not have to run through sufficient liquid.

All three bottles can easily be kept arranged on a little shelf if desired, but it is better to take off the caps after use that the rubber part may not lose its spring.

Of course, when the bag is full and the tap turned off, the tubing will also be detached instantly. The next step is to detach the tube from retort, as already insisted upon. Then turn the nozzle up the chimney till cooled down. It is best to wash out the same day, as the chloride residuum is rather destructive. Some remove it dry, breaking it up with a pointed rod, but it is much more trouble, and I think wears the retort more. When washed out, care must be taken to dry it out thoroughly, and do not forget to clear the delivery pipe and rubber tube of manganese dust, as already hinted.

The difference in wear of bags between gas thus properly purified and gas either delivered direct, or merely washed with water — it cannot be called purifying — is simply amazing.