There are two basic designs for most gas cartridge stoves used for camping: the burner assembly is fitted into the top of the gas cartridge, which serves as the stove's base, or the stove is free-standing and the cartridge is separated from the burner and connected by means of a small hose or pipe. Single burner propane stoves commonly used in food service have the gas container integrated into the body of the stove. Butane cartridges are almost exclusively manufactured in South Korea with the only exception being one manufacturer in Houston, Texas. Butane canisters manufactured in South Korea contain butane from Saudi Arabia, while the American manufacturer fills canisters with domestic butane. Gas cartridges for camping stoves tend to be costly and usually are not refillable. Single-use cartridges are considered objectionable by some on environmental grounds. They perform poorly in cold weather. Until the approval of EN 417-specification Lindal valve cartridges of 1992 there was little compatibility between different makers and systems, and cartridges for older cartridge stoves were often unobtainable. Outside of camping, single-burner stoves using cans of compressed butane gas are used for catering and other food service applications. Refillable gas bottles are heavier, and tend to be used on larger stoves intended for car camping, expedition base camps, and residential use (as for barbecues). Stoves with two or more burners that may be operated together or separately are common for use in base camp, car camping and other situations involving cooking for larger groups. The folding "suitcase" style by Coleman is the best known of this design, and is intended to be used on a flat, stable surface such as a tabletop. This type of stove may have a separate fuel tank for each burner, or more commonly a single tank shared by both burners. Multiple burner stoves generally use compressed gas, alcohol or Naphtha (also known as White gas or Coleman fuel). A solid-fuel stove may consist of no more than a metal base plate and container to hold the fuel, a set of legs to keep this assembly out of contact with the ground, and some supports for a billycan or other cooking vessel. This design is scalable, and may be used for anything from tiny backpacking stoves to large portable woodstoves. Fuel may be manufactured, e.g., hexamine, or natural, e.g., woody forest debris. Among compact commercial models, the Esbit hexamine stove burns small tablets of hexamine or trioxane in a folding stand made of aluminium or other base metal, and is a German design that dates from World War II.