Rules · Plans · Geometry · Use
Build the baptistery, and its font, with eight sides, because the eighth day, the day of the resurrection, is the day into which the baptised are born.
plan and font = octagon (8 = 7 + 1, the day after the Sabbath)
The inscription attributed to Ambrose begins Octachorum sanctos templum surrexit in usus (“an eight-niched temple has risen for holy use”) and goes on to explain that an eight-sided font is fitting because the eighth day brought true salvation to the peoples, when Christ rose from the dead. The number seven stood for the days of creation and the old law; eight for the new creation.
Octagonal baptisteries became the rule in northern Italy and far beyond: the Lateran in Rome (rebuilt as an octagon in the fifth century), the Orthodox and Arian baptisteries at Ravenna, Fréjus, Albenga, and centuries later Florence, Pisa and Parma. They stood beside the cathedral, because baptism, administered by the bishop at Easter, belonged to the cathedral alone.
Draw a pointed arch from two centres on the springing line. With the centres at the springings, the arch is equilateral; with centres beyond them, a lancet; with centres between them, a drop arch. Late Gothic added arches of four centres and the reversed curve of the ogee.
r = s (equilateral) · r > s (lancet) · s⁄2 < r < s (drop)
Classify arches by the geometry of the intrados: the number and position of the centres (one for round and segmental, two for pointed, three or four for basket and Tudor arches), or a single continuous curve (ellipse, parabola, catenary), and distinguish true arches, with voussoirs, from corbelled “arches” of oversailing courses.
A round arch must rise half its span, so arches of different spans on one springing line have crowns at different heights. A pointed arch can be given any rise at least half its span, so arches of different widths can all be made to reach the same height.
round: f = s⁄2 (fixed) · pointed: r = (s²⁄4 + f²) ⁄ s for any f ≥ s⁄2
Order the church as a single path: a colonnaded atrium, a narthex, the long nave drawn toward the altar, and the apse framed by a triumphal arch at the end of the axis.
Throw an arch diagonally across each upper corner of the square, backed by a small conch or cone; the four squinches and the four walls between them make an octagon, on which a round dome can sit.
octagon side = S (√2 − 1) ≈ 0.414 S; squinch span = octagon side
Design a rose by dividing the circle into equal sectors; fill each with the same traceried unit (a light with a cusped pointed head, an oculus above) radiating from a central hub, so that the whole is generated from one sector by rotation.
n petals · each sector 360° ⁄ n
Cite this page Chris Paton, “Eight sides for the baptistery”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/ambrose-octagon. Text, drawings and models licensed under CC BY-NC 4.0.