This gas, by reason of its lightness, will rise to the surface of the acid, and as pressure increases it will force the acid back up through the acid supply pipe into the acid chamber, until the acid falls below the perforated bottom. When the acid and zinc cease to come in contact with each other the generation of gas stops until gas is used, which relieves the pressure; then more acid descends, and as it comes in contact with the zinc more gas is generated, replacing that which has been used. This action makes the generator automatic, unless clogging with sulphate of zinc takes place. This may happen at any time if the apparatus is not cleaned after each day's use.
Cleaning the Generator.
To clean the generator in this case attach the air pump to the gas cock on the generator and force the acid up into the acid chamber by pumping air slowly into the gas chamber until the acid rises to the proper hight in the acid chamber, where it can be held by forcing a long wooden plug into the acid supply pipe. The pumping must cease when the acid rises to the proper level, or the excess pressure of air will work up through the supply pipe and cause a blow of acid.
The charging screw can then be removed and the zinc taken out and washed in hot water. Remove the clean out screw and run one or two pails of hot water through the gas chamber. This will remove the deposits of sulphate paste. The zinc can then be replaced, the screws tightened and the acid released again. Be sure and exhaust the air in the gas chamber, as previously described, by letting the air spurt out of the gas cock before connecting it to the scrubbing cup. Care must be taken not to have any lights near the generator when blowing out this mixture of gas and air, as it is very explosive.
The apparatus will never clog if cleaned after each day's work, which should always be done. The tubes should be removed and hung up over night to dry. The acid, if not spent, can be dipped out of the acid chamber and placed in jugs. The generator can then be carried to a drain and filled with hot water, which should be allowed to flow out through the cleaning screw. This will clean the zinc and wash out all the sulphate deposit. The screws may then be tightened and the apparatus left ready for the next day's use.
Fire Trap and Scrubbing Cup.
One of the most essential parts of a lead burning apparatus is a reliable fire trap and scrubbing cup. This trap reduces to a minimum the danger from explosion caused by neglecting to free the gas from air. Its use as a scrubbing cup is also of infinite value.
The action of the vitriol on the zinc produces a violent ebullition, and a small quantity of the acid is carried in the form of spray from the generator to the tubes, and, unless caught and removed, will frequently get into the blow pipe tip and extinguish the flame, making it necessary to remove the hose and hang it up to drain and dry, which oftentimes causes waste of time and annoyance.
Almost all spelter or zinc contains more or less arsenic in a metallic state. It is also found in sulphuric acid. This arsenic is released from the acid or zinc as they decompose and is carried by the force of the volume of gas to the blow pipe tip, where, owing to it being necessary for the operator to get his eyes close to the flames in order to see the reducing flame, this poisonous gas will be breathed into the lungs and oftentimes cause a fatal illness. This fact has been disputed by many, who say that it is impossible for the unit of lightness—i. e., hydrogen gas—to pick up and carry a heavy metal such as arsenic. Arsenic does not form a chemical combination with hydrogen, having a very slight affinity for it, but is carried to the blow pipe solely by the force of the volume of gas.
To prove the above assertion we will refer to Professor Marsh, who demonstrated the ability of hydrogen to carry arsenic in the following manner: If a solution containing arsenic be added to a solution of sulphuric acid and zinc, the resulting hydrogen will, upon ignition, deposit a ring of metallic arsenic upon any cold surface that the flame be directed upon. (Professor Marsh's experiment.)