There are two main sealer categories&colon; topical sealers (coatings) and penetrating sealers (reactive). Plastic bags and other packaging is often formed and sealed by heat sealers. There are primarily three types of pavement sealers. Primer differs from wood sealers in that sealers typically don't obscure the wood grain completely. Like external sealers, a plastic bag is typically used for packaging. Initially, contacts began with the arrival of sealers and whalers in the 1790s. Siloxane is the largest molecular compound of commonly available penetrating sealers. Silane is the smallest molecular compound of commonly available penetrating sealers. In recent years oleophobic impregnators have been introduced as an alternative to surface sealers. They prevented boats from landing and took clubs and rifles away from the sealers. Single chamber sealers require the entire product to be placed within the machine. The pigs were intended as a food source for shipwreck survivors and sealers. Such an epidemic may be linked to contact with sailors or sealers. Trading relationships developed between sealers and Tasmanian Aboriginal tribes. It is speculated that early contacts with sealers before colonisation had resulted in an epidemic.