Problems with a box top

[Yelverton]

This year some of my Christmas gifts were small boxes. I made 3 from heartwood pine from our family farm, one walnut box, and two cherry boxes. One of the cherry boxes hasn't been given yet, so it has been sitting closed in our dining room for a couple of days. Unfortunately when I took a look at it yesterday, the top had cupped. A day sitting open and the cup is gone.

Any suggestions on solutions? I'm guessing that the humidity in the box is higher than the house, but I'm surprised that the finish isn't sealing out moisture better. It has 2 coats of tung oil and varnish mixture on both sides.

My thought is to wipe or spray the whole box with poly (or wax?) to seal things up. I'd like to know that once it's given to its owner, the same issue won't crop up again.

Pictures are attached and any suggestions will be greatly appreciated. Pine box pictures added as well, just because the material is so great.

 

[Charlie Buchanan]

Since the cup went away after you opened up the box it was probably moist inside the box when you brought it into a dry environment. The outside was exposed to the dry indoor air and started to dry out faster than the inside. Let it sit open in the indoor environment for a while and it may be fine assuming the wood was reasonably stable and dry when you built the top. I wouldn't do anything until it has acclimated to the indoors for a while.
The pine box is quartersawn and should move very little assuming it was reasonably dry when it was built.

 

[Matt Furjanic]

I would put it in my oven at the lowest (170*) for a couple hours, then take out the cup by planning or sanding.

 

[Yelverton]

Since the cup went away after you opened up the box it was probably moist inside the box when you brought it into a dry environment. The outside was exposed to the dry indoor air and started to dry out faster than the inside. Let it sit open in the indoor environment for a while and it may be fine assuming the wood was reasonably stable and dry when you built the top. I wouldn't do anything until it has acclimated to the indoors for a while.
The pine box is quartersawn and should move very little assuming it was reasonably dry when it was built.

Thanks for the input - I've left it sitting out and it appears to be stable. FWIW, the material was fully dry - it had been sitting inside for at least 10 years before I resawed it for this project. I don't have any concerns about the pine box and the other cherry box I made was also quartersawn. It's flat as a board.