Mary,
 
The term "slag" is also used in certain glass manufacturing for molded and  
pressed items made from scrap glass. Allegedly, glass makers gathered up scrap  
at the end of the day and made various forms of toy marbles. "Slags" were a  
swirl of scrap glass, but some manufacturers in the 1930s and 1940s actually  
introduced a mix of colors to imitate scrap glass marbles. 
 
Clinker bricks were popular in the Arts & Crafts Movement for  ornamenting 
fireplaces, chimneys, garden walls, and some walkways. As you  mentioned, they 
were over-heated bricks that blackened, distorted and bubbled.  Some Mexican 
brick makers deliberately create clinkers for the American market,  as literally 
no American brick makers produce clinkers due to building  codes.
 
Some cinders sold during the 1950s and 1960s for garden ornamentation were  
actually volcanic material crushed or sifted from bombastic fields of air 
formed  lava. We used to see gardens, walkways, and even streets in the stuff. 
Barbeque  companies even sold it as ballast for their metal BBQs to keep the heat 
off the  metals.
 
Ron May
Legacy 106, Inc.