Sentence:  Good Iznik has strong colours well-contained within their outlines and a very clean, clear white. The red colour, made with Armenian <WSD>bole</WSD> (an earthy clay) should be thick and proud of the surface. 
Meaning of the word marked with <WSD> tags:  Any of several varieties of friable earthy clay, usually coloured red by iron oxide, and composed essentially of hydrous silicates of alumina, or more rarely of magnesia.