Had a hemlock slab 4” thick and 21” wide. Had been drying for over 10 years and was cupped a small amount . So had a sawmill flatten the top and bottom then cut it in half length wise. So it’s 1 3/8” thick. The slab had been rained on so I got back home and set it up to dry with a fan. So emergency situation at home I had to deal with so by the the time I got back to my boards one half of the slab was very flat but the other half was about 3/4 of the way flat with the end being cupped. So any ideas of flattening the cupped end of the board?

sounds like you got a MC imbalance OR it wasn't as dry as you expected. A lot of variables go into a correct answer BUT this is the most common I see when drying or building with slabs which usually caused from imbalanced air circulation.
1) NEVER put forced air directly to one side. One side dries faster than other.
2) ALWAYS be sure your 8" or more off floor, table, bench, wall etc, etc AND air flowing evenly into the sides as to flow thru.
3) Keep fan away and not directly pointed at surface/lumber. Drying too fast is just as harmful.

I think you can save IF corrected soon by elevating (#2) and slow balance the MC by indirect air movement..
Sometimes reslicing a slab imbalances as internal MC is 95% of the time different than external and needs time to rebalance.
Your rained on is also a concern BUT usually can be rebalanced IF proper steps done.
I recently proved to a builder how shop MC/RH's can create havoc on warpage by not air spacing above benchtops enough. I AD the pecan lumber over several years and was relatively flat at purchase. He started his slab tables then called re a concern of slightly warping. After consulting and going to his shop I seen and explained 3/4" air space is nearly NO air movement. We raised to 6" and let acclimate a week and it flattened back to original flatness.

I think the rain then a more direct towards one side air movement (without further info) and imbalance air flow may have been your culprit. 98% chance it will reflatten once properly acclimated evenly.
My website hasn't been updated in a long while BUT has pics to show my sawing and drying knowledge.
I hope all turns out well.
Tim

It went from 4" thick to 1 3/8" thick to make it flat? They really had to remove 2- 5/8" of thickness? No surprise it cupped again!

I agree Rich BUT that also has to do with how it was sliced and the wood's grainage. BUT on average you're correct as most would slice one cut from one side....which falls into a stress issue. As I said before NOT enough info for a definite answer.

not nearly enough info. My jaw dropped when I read how much the thickness changed. Somebody made a lot of money off this guy selling him 16/4 when it ends up as 8/4.

Rich C. for being so critical about every post on this forum, please read and understand the OP's post clearly before responding.
He states above, "So had a sawmill flatten the top and bottom then cut it in half length wise."
Geez....

Update, the hemlock 1 3/8” boards are now flat. Will be using my 18” sander to complete the flatness this week. I appreciate the first’s guy’s response to me. Haven’t done much with reclaimed hemlock before and it sounds like some others had limited experience with hemlock. Hemlock can be difficult to work with like hand planing it one can cause rip outs from the wood even. So hopefully sanding should be much of an issue.

Too afraid to use your real name Captain? Just prefer to pick on me anonymously ? A lengthwise rip can be in width or thickness. At least in my shop that is the way it works.