Wood shrinks much more across the grain than with it with annual changes in humidity (see picture in linked article below). With such a wide solid wood frame you are pretty much fighting a losing battle. If you make the joints tight now they are just going to open again when the humidity changes. No amount of clamping will change that. Most woodworkers will go to pretty great length to avoid plain miters for this reason.

If I needed to make this design I'd make the frame from something dimensionally stable like plywood or MDF and veneer it.

article-- why miters move

The wider the stock is, the more that seasonal movement will exacerbate the problem. As the wood dries/shrinks, the 45* cut face becomes convex, hence the gap. If it expands, opposite happens, the mating edges become concave, open at the outside corner. Miters are not stable woodworking joints.

Casey

lovely frame. Can you fill with wood putty and color match?

That was a really interesting article, thanks! This makes
sense. Since it’s fully joined flush at the inside end, I guessed there was
some kind of shrinking going on. That’s why I wondered if clamping could even
work.

Take a look at this shot of the back of the frame:

This uses the pocket hole system also referenced in the
article. Where those screws are placed, it is tight; where they are not, it opened.
I am not a carpenter and just do stuff for my own house, so it does not make
sense for me to get something like this.

I COULD fill it and match it, I just figured it would pop
out in time anyway.

Just out of curiosity: is this a wood used by woodworkers / artisans? Or
a wood used by makers of mass –produced pieces, like stores such as Pier One would
carry? When I got it home and noticed the screws, I thought “artist made” rather
than mass produced. It would be fun for me, as the tile I am going to mount in there
is a handmade one I bought in Seville Spain last
year.

It's only free in the Brazilian forests, the cost comes mostly from the shipping, add a little bit for the sawing, drying, profit. It's definitely in the "premium"category, not "super premium".

Casey

Now that I know I can't clamp the corners together (thanks for saving me time and frustration) , what about filling? Am I correct in thinking it would only pop out later?

You could try filling...you really don't have any other options. You can't fix the bowing that has happened.

Yes, worth a try. It won't be any worse off by making the attempt.

If you fill the gaps, it will probably look like you filled the gaps. You might consider taking some thin strips of a contrasting color wood and apply them over the joints to highlight them rather than trying to make them invisible.

JM$.02W

randy-

I have often used a similar method to what you are suggesting when trying to mask a flaw in one of my woodworking projects, but, in this instance, I think the gaps are small enough that just filling them with as close a match of wood filler as possible may produce adequate results, The OP could always try filling the cracks first, and then move on to your more sophisticated suggestion if unhappy with the first approach.

I'm really good at camoflaging fills, so I'll go that way first.

I did fill them, and matched the color, more or less, so I will see how it goes. It they do pop out, I had not put much time into it.

Looks great. I doubt it will pop out. And the casual observer won't realize you made a fix even if the cracks open up again.