| Lbs. per square inch. | Temp. Fahr. | Temp. Cent. |
| 45.512 | 275° | 135° |
| 52.548 | 284° | 140° |
| 60.442 | 293° | 145° |
| 67.408 | 300.2° | 149° |
The illustration, [p. 64], shows a vulcanizer of modern type made for rubber stamp work. In some recent vulcanizers the water and steam are excluded from the vulcanizing chamber, being contained within double walls forming a steam jacket and maintaining a constant heat within the chamber. These illustrate a point that has been much misapprehended, namely that curing is independent of pressure or atmosphere. Because vulcanizers have generally been filled with steam at high pressure many have supposed that the steam or pressure had something to do with their action. The fact is that it is only the heat due to the steam at such pressure that is instrumental. Steam is a very powerful radiator and absorber of so called radiant heat. For this reason an atmosphere of steam maintains all parts of the vulcanizer at an even temperature and is to that extent advantageous. Its presence and the pressure it generates are not by any means required for vulcanizing. Its pressure is entirely without effect.
Steam Jacket Vulcanizer.
To use a steam vulcanizer, water is introduced, the article in the press or mould is placed in it, and the top is secured. Heat is then applied, best if on the small scale, from a Bunsen gas burner gas, or oil stove. Either the pressure gauge or thermometer may be watched, and the flame turned up or down to keep it at the proper temperature.
Moulding cannot be executed in the ordinary closed chambers. The press must first be heated to the temperature of boiling water or thereabouts and the moulding is then effected by screwing down the mould screw, upon the sheet and matrix. It is then placed in the vulcanizer and cured.
The manufacturers supply gas regulators which automatically regulate the gas supply. These are worked by the steam pressure. If any one wishes to study the practical manipulation of small steam vulcanizers he can see them in use at any dentist’s office.
There is no need of a steam vulcanizer for ordinary stamp work. The hot press system already described answers every purpose and is in use by the most advanced manufacturers for thin sheet work. But if a wooden moulding press is used then it must be heated in a vulcanizer or some kind of oven or hot chamber.
A very simple and reasonably satisfactory oven or air bath can be made from a flower pot and a couple of tin plates. A plate larger in diameter than the mouth of the flower pot forms the base of the apparatus. This is supported on a stand over the gas lamp or other source of heat. A smokeless flame or one depositing no lampblack should be used. Alcohol or a kerosene oil stove illustrated on [page 59] are excellent. On this plate a smaller plate is inverted, which latter must be so small as to be surrounded by the flower pot and to be included within it when the pot is placed over it like an extinguisher.
