
1) Clay
Found on
https://www.crosswordclues.com/clue/fireclay

(from the article `Toft, Thomas`) Toft was the first to add aluminous shale, or fireclay, a clay that can withstand high temperatures, to the paste for his earthenware. His work is ... [4 related articles]
Found on
http://www.britannica.com/eb/a-z/f/28

A type of clay that is capable withstanding high temperatures of a furnace or kiln. Fireclays are often found in connection with coal seams, so collieries often had a brickworks.
Found on
http://www.encyclo.co.uk/local/20766

A type of clay found in association with coal and shale which when fired is particularly resistant to heat. Fireclay was formerly used to make facing bricks, ornamental features, ridging and below-ground drainage pipes and gullies. Fireclay bricks are yellowish, or whiteish. Salt-glazed fireclay pipes and other products are shiny and brown. Relate...
Found on
http://www.encyclo.co.uk/local/20938

A variation of vitreous china having greater amounts of quartz and feldspar in the clay material beneath the vitreous surface. Fireclay is heavier (30%), denser, and thicker than regular clay. These properties insure smooth, flat surfaces on large fixtures which is not achievable with standard vitreous china.
Found on
http://www.tjader.com/glossary.html

[
n] - a heat-resistant clay
Found on
http://www.webdictionary.co.uk/definition.php?query=fireclay

Refractory clays which withstand high temperatures, are yellow or grey in colour and are used for parts of kilns and bricks. They are too coarse for use in most domestic pottery.
Found on
https://ashbrook-ceramics.co.uk/pottery-glossary/
No exact match found.