Native trees of Canada, by R C Hosie in notes under red maple (acer
rubrum L) . Maple lumber can be identified as "hard" or "soft", by
applying any solution of ferric salt to the sapwood---- blue stain,
soft, green stain , hard maple. A drug store may have ferric salts,
which can be dissolved in water and applied with an eye dropper.
I used this method with good results when trying to sort out
mixed piles of red and sugar maple logs while trying to sort out a
dispute on who cut what and on whose property.. I have not used it on
sawn lumber, obviously some sapwood must be present .

Ken