The Mortise and tenon is marginally weaker, but tends to fail in such a
manner as to leave the furniture in one piece -- albeit a bit wobbly.

It is a difference of 3 biscuits fail at 2700 lbs. -- 2 biscuits at 2700
lbs. -- and M&T at 2600 lbs of force..


With no scientific or laboratory test results to back this up I will say the
mortise & tenon is stronger than dowels or biscuits. We did a home shop
test with the following results.

We used alder as the test wood. Making a face frame 12"x15" out of 3/4" x
3".
We made one frame using biscuits 2 per joint, one with dowels 3 - 3/8" x 3",
one with pocket screws (2) and one with MT, two corners were cut and two
were loose tenons.
All were glued with the same yellow glue and allowed to dry for 2 weeks.

Then using the shop press we put each into the press in a diamond shape and
pressed away.

The pocket screws failed first followed closely by the dowels and then the
biscuits. The MT joints were significantly tougher. However, since we were
surprised at how quickly the previous 3 joints failed we tried something
new.

Making more frames - same as above, we tried different clamping methods.
Other than the pocket screws, we clamped in the typical method holding first
then we added clamps to the actual joint - think compressing the mortise
onto the tenon. Wow, what a difference in strength!

Using the same (un-scientific) method of destruction, we found that again,
the screws failed first, then the dowels but with greater force than before.
The real difference was with the biscuits and MT. After pressing much
harder, the biscuit joint failed. The MT (not much difference between the
loose and fixed tenon) really took quite a bit more pressure before they
failed.

Summary, from weak to strong, pocket screws, dowels, biscuits, and way
stronger, mortise and tenon. And for real strength, clamp in both
directions!

Dave



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