For many years I have heard people using things like Salt and Sand as a medium between 2 boards being glued, to help eliminate the slippage that happens when we try to glue boards together. I have never used the technique but recently I have had a flurry of people asking me if the using Salt as a grit when gluing boards together, does that weaken the joint?
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It sounds plausible that it might, but it could also strength the joint ... or it might do nothing noticeable, but I will never know if I don't test it, so that's exactly what I set out to do ...
There are a few ways of testing joint strength, and I could use a press and get some readings with and without, but in the end, all we really want to know, in fairly simple terms, does the glue or any other grits really have much bearing on the strength of joints.
I decided to use both hardwood (Red Alder) and Softwood (Fir) and a variety of grits to test each combination to see how they compare.
My idea for a simple test is to see if the wood will fail before the glue joint, anything more than that I really don't care about.
The grits I would test were Play Sand (this comes in bags from the hardware store, I use it to spread on icy sidewalks in the winter), White Sand, this is a common product found at most paint stores and is used to sprinkle on painted steps when the paint gets tacky, to make them less slippery and finally ordinary Table Salt.
I started off by cutting 4 blocks of the hardwood and the softwood that I could use for testing and then cut small angle cut along the top of each one, big enough that I could get the claw from my hammer into to try and pry the woods apart after gluing.
I began with the hardwood and put an equal amount of glue on all the marked areas both on the small blocks and on the long main piece, then sprinkled a small amount of grit on each one Play Sand, White Sand and Salt and of course, one was left as Glue Only for comparison.
I did the same thing on both the hardwood and the softwood, then let them both sand in clamps for 24 hours.
After 24 hours I began leveraging off each of the blocks, starting with the Glue Only block, then moving to the Play Sand, then the White Sand and finally to the Salt.
To my amazement, ALL of the grits were stuck firmly together and even the Salt on the hardwood was glue firmly. Every block that broke off, broke the wood before the glue joint ... with one small exception. The salt joint on the softwood, although it did break some wood, much of the block that broke off was along the glue joint.
To re-test this, I made another glue joint and copied exactly what I had done on the previous joint, but this time I made sure the clamp was firm (I am not positive the previous softwood / salt test had the bar clamp on as tight as it needed to be)
The last test, the Salt on the softwood resulted in exactly the same outcomes as all the previous tests - the wood failed before the glue joint, which tells me that using Salt, or any fine grit for that matter, doesn't matter to the strength of the glue joint - the wood fails before the glue joint fails so anyone who wants to use this technique, as long as they sprinkle a small amount of grit on their boards and clamp firmly ... should have no problems with slippage or with using the grits.
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Copyright Colin Knecht
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