The Making of the Glencairn Methuselah Window: Page 20

250 grammes 1 sand pulverized 4 red lead
Cullet 1-4
12½ grammes burnt modelling clay
12½ Manganese Dioxide ground finely

The above bubbled up a lot and when it reached <left blank>?F was removed by putting a poker into it and bringing it out in gobs

8 of the above Manganese flux ground in pebble mill
1 black copper oxide then ground on glass slab
This makes Mediaeval Black – a slightly warm Black