Cones (noun) refer to cone- or rod-shaped objects made of specific formulas of refractory and fluxing materials which indicate specific temperature and heat-work ranges within the kiln as it is firing.
Cone (adjective) is used when indicating which temperature/heat-work range is being referred to. Cones are known by numbers; numbers that start with a zero (such as "cone oh-eighteen") are lower in temperature than those which do not. There is no "cone oh".
I need to buy more "6" cones. (noun)
I'll be bisque firing to cone 08 tomorrow. (adjective)