  In 1989, Vaughan Jones introduced spin models and showed that they could be
used to form link invariants in two different ways--by constructing
representations of the braid group, or by constructing partition functions.
These spin models were subsequently generalized to so-called 4-weight spin
models by Bannai and Bannai; these could be used to construct partition
functions, but did not lead to braid group representations in any obvious way.
Jaeger showed that spin models were intimately related to certain association
schemes. Yamada gave a construction of a symmetric spin model on $4n$ vertices
from each 4-weight spin model on $n$ vertices.
  In this paper we build on recent work with Munemasa to give a different proof
to Yamada's result, and we analyse the structure of the association scheme
attached to this spin model.
