The traditional square network of kumiko in shoji are relatively straightforward, with lap joints at each intersection. The triangular, or diamond pattern, however, is a fair bit tricker as each node of the lattice involves three members to join. I don’t know of any written information in english on the techniques for this — though there are some sketches in Nakashima’s Soul of a Tree. I pestered a couple of expert shojimakers in the US while puzzling out the details, and I thought I’d put up a not-so-brief summary of the basic techniques and joinery.
I’m going to try to document this as well as I can without taking 2 hours of your time, but please be forewarned that it may be a bit confusing at first. It may help to have a look at the final piece first, to get a sense of what we’re working toward. Here is a roughly 24″ x 7″ section of diamond-pattern kumiko that I’ll be using in the final ranma:
In order to make the grid work, there are three different members of the lap joint that have to be employed – one for the horizontal pieces, one for the ‘left leaning’ pieces, and one for the ‘right leaning’ pieces. Here are my test sample pieces:
Second – these joints are really made to be assembled ONCE and once only. Assembling them compresses the fibers at each joint, giving you a fairly nice fit, but they will never go together that cleanly again if you take them apart. If you look at that final test joint photo, you can see what I mean. That piece was put together and taken apart twice before this picture, and you can see on the left and right edges of the joint where the piece has been somewhat crushed, making it feel ‘not so fresh’.