As written above, all the bottles were salt-glazed. Salt glazing was introduced gradually during the 16th century in Germany.13, 9 The sources of information we had about this process were contradictory. One stated that: 300kg of salt was needed for 10,000 bottles and that after 45 to 50 hours, or up to three days of firing, when the kiln temperature had reached 1150 to 1200 degrees centigrade, 80% of the salt was thrown into the bottom of the kiln and 20% through the draught holes in the top; the salt combined with the salicilic acid in the red hot clay.10 Some sources state that when the salt was thrown in towards the end of the process, this produced the ‘orange-peel’ effect found, for example, on German Frechen stoneware.1 By contrast Gaimster 9 suggested that the ‘orange-peel’ pitted nature of Frechen stoneware was caused by the dense quartz-sand in the clay. Of course the pitting effect may have occurred at any time during the process, but only became obvious at the end of firing. The source material on the later, 19th century Apollinaris bottles in particular, suggested that after the salt was thrown in, the kiln needed to be fired for a further 8 to 10 hours in order to achieve a more even glaze.10
Nienhaus, stated that wood always had to be used when glazing was taking place, as coal did not provide the higher temperature needed for a smooth, even glaze.14 However notwithstanding, in England in the 18th and 19th centuries coal was used to fire stoneware kilns, which apparently, so it is said also provided a more even temperature. 9 Green, when describing Dwight’s Fulham Pottery in London, suggested that from the end of the eighteenth century coal was used during firing and that this was the cause of differences in the glaze colour and may have been responsible for a darker glaze than before.20 He also suggested that when using coal rather less mottling was evident than previously. It seems that the subject of the effect of different fuels on salt-glazing needs more research.