The whale oil burner commonly consists of one or two tubes made of tin set in a base of brass, tin or pewter. They may be threaded (designed to screw into collars), or to drop-in (usually on a tin and/or cork disk). Whale oil burners characteristically have a large portion of the wick tube that extends downward into the oil fount. Because whale oil was quite thick, especially at cold temperatures, this design helped to transfer heat from the flame to liquefy the oil. Thus, it was more easily drawn up the wick. Adjustments to the wick are generally made though round or slotted holes in the wick tube with a 'pickwick.' Early whale oil lamps frequently had coarsely threaded pewter burners and collars. As advances were made, more burners were made of brass and the threading got finer. Because pewter is so soft, care must be taken not to cross-thread these old burners or force them onto a collar with different sized threads.