I thoroughly enjoyed Robert Baldwin's article on the brickwork construction at 'The Landmark', Ilfracombe (aj Brick Bulletin, 29.10.98).
I note that 18degrees was found to be the maximum inclination possible without the brick sliding on wet mortar.
The description is similar to accounts of the construction of the dome of Florence Cathedral. Brunelleschi's use of herringbone brickwork seems to have facilitated the construction of the dome at steepnesses exceeding 18degrees. (He alternated stretchers and bricks on end.)
A later drawing by Antonio da Sangallo the Younger, entitled, 'Rounded vaulting of medium-sized bricks as it is made without centering in Florence' is in the Uffizi (drawing no 900A). It shows the introduction of a brick- on-end after every five stretchers around a dome's perimeter. The bricks- on-end form the jambs of apertures which are infilled with another brick- on-end and further stretchers. As the steepness begins to exceed 18degrees, the infill-bricks, in sliding inwards, reach a point where they can slide no further, having wedged against each other like radial voussoirs of arches spanning between the bricks on end. (See Benevolo L, The Architecture of the Renaissance Vol 1, translated by Judith Landry, London and Henley. Routledge and Kegan Paul, 1978).