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.)
It will be seen from the above that it is imperative that the operator use a scrubbing cup and see that it is properly filled with a solution of blue vitriol. The ordinary impurities of hydrogen generated in this manner are sulphur and carbon, which should be removed if possible.
The actual use of the scrubbing cup is to catch the above mentioned spray and precipitate to some extent all other impurities contained in the gas, and produce gas sufficiently pure for lead burning.