Janet R kindly sent me these references after I puzzled over a video which showed two bricks protruding from a firing walled up kiln. I thought it may be so they could check how the firing was progressing, but wasn't sure quite how it worked in practice:
'There was also a peep hole at each end of the kiln near the top and they were plugged up with a brick that you could just draw out. When you looked through, you knew where your top was. You can imagine all the heat that was coming a front, you'd no eyebrows and if you were a moustache man, no moustache! You manipulated these loose bricks with two bits of wood in your hand, and held it to keep the heat away from your face so you could look down into the kiln.'
p.33 The Barton Area Brick and Tile Industry: a personal view. By Ernest Coulam, edited by Karin Negoro from interviews recorded in June 1990.
'..as the front wall rose a small aperture was left above the top of the kiln gap but just underneath the level of the arch, near the top. This was above the height to which the contents would reach when set but at such a point as to allow a sight along the kiln contents under the top of the arch. The purpose of it was that it could be filled in with bricks and pug after allowing the burner to withdraw a brick and take a look along the top of the contents when the kiln was burning. When he was satisfied with what he saw the brick(s) were replaced and pugged in. A similar opening was built in the rear wall...one of the bricks was marked with a cross or something and always left protruding about 3" so that it could be easily withdrawn for this purpose.'
pp.30-31The Barton Area Brick and Tile Industry: from turning-out to management, open top kilns, arch kilns and downdraught kilns, brick and tile making machines, building and burning in Barrow Haven and Barton. Copyright E. Coulam and C.H. Watkinson, May 2000. Re-produced and re-typed by Karen Spavin July 2002.
Janet purchased one booklet from the Ropewalk in Barton-on-Humber which she says also has some displays and exhibits on brick and tile. The other was bought from the Environment Team at North Lincolnshire Council at Scunthorpe.
Many thanks for all this Janet :-)