Medieval English Baptismal Fonts, the Basics.

Cox and Harvey categorize the fonts they encounter as coming from three particular eras, the Saxon era (after the Romans but before the Conquest of 1066), the Norman era (which seems to be from 1066 sort of though the reformation), and 'early English' (by which I believe they mean post-reformation, or even post- Elizabethan). 

Of the fonts mentioned, the lion's share are put into their 'Norman' category.  But fewer remain in either the Saxon or Norman category than should.  The puritanical zeal of the Reformation caused many medieval church fonts to be lost, defaced or destroyed.  Whatever could not be melted down or sold to enrich Henry VIII and his "visitors" was treated with callous distain.  Reformer fundamentalists forbid the use of the beautiful fonts— many of which were subsequently destroyed—and demanded that a simple basin be used instead.

Considering the tumult from the Conquest, and the ages before the Reformation, it is not so surprising that few Saxon fonts remain.  Of those Cox and Harvey mention, three are particularly interesting.  The font at Wilne, for instance, was made from an old Saxon cross (those large, ornate church-yard crosses particular to the Saxon churches), and two other fonts were inscribed with pre-Norman lettering which helped to date them (from page 166):

"In two cases the pre-Norman characters used in font inscriptions prove their Saxon date. The inscription on the circular  font of Little Billing, Northants, engraved  by  Paley, runs round about a  third of the circumference of the bowl, with two horizontal lines of Anglo-Saxon lettering ; it reads as follows :—

Wigberhtus artifex atque cementarius hunc fabricavit,
Quisquis suum venit mergere corpus procul dubio capiat.

Wigbert the artificer and mason made this (font),
Whoever comes hither to dip his body, let him take it (Baptism) without doubt."

Font types and materials, excerpts from pages 160-1:

"In England the use of a baptistry separate from the church never prevailed.  In Cornwall there are a few interesting instances still extant of Holy Springs, possibly used as baptistries, and protected by chapels ; and the same are to be found in Monmouthshire as well as in Wales.  But the almost invariable rule in these islands [in the British Isles] seems to have been to place a font in the body of the church ; in all events this custom was universal amongst us in post-Conquest [after the Norman conquest in 1066] days."

"The font itself was as a rule [made] of stone, and it was usually lined with lead [even the wooden ones], save [except] in some of those instances where an [a water] imprevious stone, such as granite or Purbeck marble, was used.  Wooden fonts were occasionally in use in those early days, but they were always considered irregular, and in later times uncanonical [unofficial]."

Of wooden fonts, Cox and Harvey say that in 1297, the West Lee church in Essex had a wooden font. A font carved from a single, solid block of wood was used at Efenechytd in Denbigh.

Fonts could also be made of cast metal, such as bronze, or gilt [given a thin outer layering] with silver. Precious metals were rare, though.  Of the metal cast fonts, most were made of lead.

On Cleanliness, excerpted from pages 164-5:

"Fonts were ordered to have covers and to be kept locked for the double purpose of cleanliness and for checking the use of the water for superstitious purposes.  The Bishop of Exeter, in 1287, ordered that each parish church was to be furnished with a baptisterium lapideum bene seratum [a cover].  Archbishop Winchelsea, in his visitation of 1305, inquired  whether there was a fontem cum secura [lock].  A provincial English Synod, held in 1236, provided that the water was to be changed every seven days.  The rubric of the first English Prayer-book provided for the change being made once every month ; the Scottish book, of 1604, ordered the fort-nightly renewal of the water..."