Most condoms are made from natural rubber, the sap from rubber trees (latex milk). Originally, the rubber was cut up into small pieces after congealing and solidifying in contact with air and was then liquefied again in a petrol solution. This process was complicated and extremely dangerous, however – explosions were not unusual at the production facilities. A search was therefore made for a process to concentrate latex, i.e. to reduce its water content and thus to make it easier to transport while keeping it liquid at the same time. The solution that was, finally, found was to heat and evaporate it with the addition of ammonia, that stops the latex from congealing. When the natural latex concentrate is heated up again during further processing, the ammonia evaporates as a colourless gas that has a penetrating smell.
About 700 condoms can be produced from one litre of latex nowadays. Glass moulds are dipped directly into the liquid latex milk to do this. The latex milk consists of the sap of the rubber tree (60 per cent), water (38.5 per cent) and sulphur / zinc oxide (1.5 per cent). Following brief immersion in the milk (the temperature of which is 20 to 30 degrees), the moulds are dried and are then covered in a fine rubber film. Further immersion baths and drying phases follow, until the condom has reached a thickness of between 0.03 and 0.08 millimetres in its unfinished state. Brushes produce the rim at the open end. The condom is then vulcanised at 110 to 130 degrees in hot air chambers before it is pulled off the glass mould. A washing operation now follows. The water contains silicone and powder, so that the condoms are coated to make sure they are no longer sticky. After this, they are dried at about 85 degrees. They are dipped in an electrolyte solution to check whether there are any leaks. Any hole in the condom reduces electrical resistance. If the lamp on the test station goes on, electricity is flowing through the condom, i.e. it has a leak and is ejected. The manufacturers of condom brands make further random material checks (leak test, inflation test, elasticity test, microbiological purity test). In the tear resistance test, for example, a condom has to survive stretching to seven times its normal size without any damage.