moisture gradient in 9/4 slabs

[Joey Grimes]

What is an acceptable moisture gradient shell and core for 9/4 hardwood slabs dried in a dh type kiln ?

[GeneWengert-WoodDoc]

Is this during drying or at the end of drying?

I have never seen any specifications, research, or publications about acceptable shell to core MC gradients during drying.  The reason is. partially because  a 30% MC gradient when drying red oak initially is of no concern, as most of the gradient is with cells that are losing free water and have not begun to shrink...cells do not shrink until they are under 30% MC.  Further, as the wood dries and the shell is down to 20% MC average, the core can be well above 30% MC, but has not started to shrink, so really the maximum gradient in this case would be 10% MC, as cells do not shrink above 30% MC. 

A shell to core gradient, in addition to being misleading, is not sensitive enough.  This is because right at the surface you can have too much surface gradient, even though most of the cells in the shell are still over 30% MC.  In just one day, you can surface check oak, but the shell and core will be over 60% MC with a very small gradient between them.  Many surface checks never go deep enough to be in the core, so the shell-core gradient is again not sensitive enough.  Even if we could get smaller gradients within the wood, there is no standard for the acceptable size of the gradient.  Such a standard, if ever developed, would change with MC, temperature, species, thickness, heart-sap, etc.; it would be too general to provide information for controlling a kiln.

Also, can you imagine measuring shell to core MC gradients every day?  I am not sure how we would do that, as moisture meters would not be reliable enough, so we would have to cut test pieces and then would probably not get MC results for 12 hours or more and that delay could be too long to be useful.

For this reason, we prefer to use the rate of drying and not a shell core gradient.

[Den Socling]

Joey, If you have surface checks, the gradient is/was excessive. If you don't have surface checks and a meter reads 10% or less, the gradient is not excessive.

[Joey Grimes]

No surface checks a mixed load of maple elm persimmon sassafras  slabs .My kiln samples were showing 15 % my moisture meter read 6-8 % final setting on kiln 120db 75wb it was at 120db 80-85 wb for last 3 days and no water .

[Den Socling]

I would raise the DB a tad. There is still a little more bound water than you want in a slab that you are calling dry.

[Don P]

Joey, If you have surface checks, the gradient is/was excessive. If you don't have surface checks and a meter reads 10% or less, the gradient is not excessive.

Putting it like that, I wonder if a strain gauge on the surface of a sample board could somehow play with the kiln control?

[GeneWengert-WoodDoc]

This is a great idea.  It was researched some years ago (I think his name was Dr. Hale from the university of NH.) and had limited success, partly because the wood relaxes with time and this was hard to incorporate in the strain gage and partly because we do not know the strength of the wood surface very precisely, so we cannot determine the precise maximum stress allowed.  The idea is really good.