Found at the old Glenboig Union Fireclay Works site. It could read Gartcosh or Gartcraig but because of the find site, I will assume it says Gartcosh. Sil probably reads Silica. Gartcosh Silica? Gartcosh Fireclay Works, Gartcosh, Glasgow. .
Found by Eddie McLean in Glasgow
Find location would suggest Scottish origins but….
The 2 most likely Scottish makers are
Bonnybridge Silica and Fireclay Co Ltd, Bonnybridge
Eglinton Silica Brick Co. Ltd, Dundyvan, Coatbridge
However I have discovered this reference which is also a possibility however I am sure many companies will have stamped their bricks silica.
1951 – OUGHTIBRIDGE SILICA FIREBRICK CO. LTD., OUGHTIBRIDGE, NR. SHEFFlELD. T.A., “Silica, Phone, Oughtibridge”. T.N., Oughtibridge 40804. Established 1856; Incorporated 1918.
Silica bricks and cement; Serpex (magnesium silicate) bricks; magnesite cement ; silica, magnesite, chrome-magnesite, chrome bricks and cements; parging cements; ground ganister; etc.
Trade Names: Silica; S.P. Silica; Serpex.
Managing Director: S . Brooke.
Below – this similar mark was found in Fife by JC. Note the thumb print to top right.