Are Biscuits Just Gravy?
Issue: Issue 145
Posted Date: 3/14/2006
Since biscuits arrived on our shores, the argument about whether they are a valid replacement for mortise and tenon joinery has continued, and today is no different. Editor.
"I was looking through [the book] Practical Design Solutions and Strategies and read John Wagner's article 'Choosing the Strongest Joinery for Doors.' To make a long story short, he found that a two-biscuit joint failed at 2,700 pounds of pressure in his test, while loose tenons and mortise and tenon joints failed at 2,600 pounds. Three biscuits raised the failure threshold to 3,000 pounds. Do you think this translates into furniture? For example, I am thinking of making a small writing desk. Should I bother with tenons when I have a perfectly good biscuit joiner languishing in a drawer in the shop?" Jay
The first to respond was suspicious. Editor
"I get suspicious of tests that don't make sense; this one doesn't make sense to me. A mortise and tenon has much larger glue area and much more wood between the parts, so it is hard to believe it is weaker than a couple of biscuits. I will continue to use mortise and tenons." Charlie
The original poster responded. Editor
"Actually it does make sense, because they were very careful about controlling variables. They standardized as much as they could; for example, 'the glued surface area of the dowels used in one joint was the same surface area as the loose tenon or mortise and tenon used in another.' So, in 'real life' the actual tenon might be much longer or larger than those used in this comparison. Thus, perhaps, leading to your conclusion that the tests don't make sense. They also compared the 'dramatic failure of the biscuit joints' with the gradual failure of the mortise and tenon. They note that even when the joint fails, the pieces are still attached to each other. They conclude that 'the best joint' is the loose tenon principally because even while it opened up, it still showed resistance to deflection and overall good strength, but the question still remains, if I were going to attach a table rail to a leg, would I be gaining that much more by making a loose tenon or mortise and tenon joint rather than using a couple of biscuits? After all, isn't a biscuit just another form of a loose tenon, albeit a bit shorter and thinner?" Jay
Another poster insisted that the test itself was irrelevant because people don't typically make tenons, loose or integral, as small as biscuits. Editor
"I think I would be more inclined to compare a biscuit to a spline, and not a loose tenon. A loose tenon still follows the mortise and tenon joint formula, ergo it has to end up physically much larger than a biscuit. The proper comparison should have been a properly sized tenon, versus a properly located biscuit. Mike
Another poster was concerned about appearances if the joint failed. Editor
"I don't like the loose tenon idea because to me, it's just a big biscuit. I much prefer an integral tenon, particularly on an apron to leg joint. If I sell a table and someday it comes apart for whatever reason, I'm gonna feel a lot better if the buyer doesn't see a biscuit." Kirk
Was there a resolution to this question? No, and we doubt there will be. It's this difference of opinion that makes horse races, and woodworking, so interesting. Editor