Glass was used for many decorative and practical purposes in the Viking Age. The image above shows a glass kiln used for melting cullet (raw glass) which is basically made of sticks and twigs encased in well grogged clay, with a lot of straw or hay bound into it. The whole affair is then 'fired' inside and out to harden the structure.
The form is more or less in the style of a crude chimney. The thick bowl has a blow-pipe of clay to feed the air in at the base via the bellows. The bellows are not anchored down so that they can easily be slid in and out of position in the clay pipe. This helps to regulate the temperature and also prevents the wooden union piece at the front of the bellows from being burnt by the great quantity of heat in the kiln. Some experiments have been done to see if the bellows work effectively if the wooden union is anchored a little way from the clay pipe, with just the force and blast of air being sufficient to fuel the kiln. This ensures that there is no physical contact with the kiln preventing damage to the bellows, which are difficult to mend. It remains to be seen if this is an improvement or not.
From the bowl upwards, the structure gets slightly thinner and culminates in four struts that come together leaving four loops. They create the roof of the kiln which acts as the floor for the block of marble. The excess heat in the kiln warms the marble ensuring that when the glass is rolled on the smooth block, it doesn't suddenly get cold and harden. Turkish bead kilns that are still in use today are of a similar although larger construction.
The loops give adequate access for the worker to heat the glass on the pontil rods, especially on breezy days when the cool air can harden the glass very rapidly, giving you little or no time to work it. The whole process is fraught with heat dangers, so expect to lose your eyebrows.