Making custom fit, hardened leather arch support inserts

My girlfriend has painful, high arches. Custom fit orthotics are expensive, I’ve heard $350. There are some kits on Amazon for less, but I’m dubious on the quality. I designed a method to build them with a bit of ingenuity. Start with some gauze and plaster of paris. 1 roll of 11.4 cm x 2.7 m was more than enough for one set of inserts, and one 4 pound carton of plaster was just right.

Have her sit back and put her feet up on a chair with her heels hanging over the end. Put a tarp under the chair. Mix up the plaster, cut a strip of gauze maybe 18″ or so, smush it in the plaster, put it on her feet. Continue applying strips to achieve good coverage and thickness. Give it 20-30 minutes to set, take it off.

Line the negatives with plastic wrap.

Fill them with more plaster and give them some knocks to settle it in there. Let it sit overnight.

The next day, take them out. It might be a little tricky. It’s OK to bend the edges of the negatives out a bit to get them loose. Sand off any big bumps on the bottoms and sharp edges on the tops. It’s OK if there’s lots of small imperfections. They’ll still feel and work fine.

Outlines of the insert and rise of the arch are marked on the inside of the negatives.

The excess is cut away, and the negative flattened to create a template for the insert.

The template is transferred to tracing paper for single-use patterns, and foam sheet for master patterns.

10 oz (~3 mm) vegetable tanned cowhide is cased on the flesh side, then the pattern impressed into the leather with a stylus.

The patterns are cut out.

They’re next skived (thinned) on the flesh side. Full thickness at the arch for maximum strength, tapering to nearly feathered at the edges. The surface is lightly sanded.

I apply my X1 hardening technique. In short, the leather is preheated to 150F. Stearic acid is melted, then cooled to 150F. The leather is immersed and then placed back on the heat. Temp is raised to 200F and held there for ~45 seconds. The leather is removed and immediately pressed into shape. After cooling, the result is an extremely hard piece that offers considerable support. It’s highly resistant to moisture, punctures, and cuts. Despite the hardness, it bends when flexed instead of cracking, even at below freezing temperatures. It won’t soften in warm weather either, due to both the thermal hardening effect and the high melting point of the stearate.

The leather is then pressed by hand onto the forms, and vacuum sealed with a FoodSaver device. This is tricky as all hell to line up right. I had to cut a V notch in the back due to bunching of the leather, and molten stearic acid is slippery. I pressed the seal button a few seconds at a time, and continued guiding the leather into shape as the bag pressed around it until the machine finally got all the air out and heat sealed the bag.

The inserts are given a couple of hours to cool and harden and are removed from the bags and forms. Despite my efforts, I couldn’t avoid all the bunching. No matter. After removing the excess stearic acid with a heat gun and rag, I’ll bring it to the belt sander and take off the bumps.

Finished. She says they’re very comfortable and definitely help the pain. I understand these are not to be worn continually or dependence can develop. Best used just to relieve pain from high arches. Her feet hurt less now. Let’s go walk the dog.