I am making a red oak carving blank which will be inside a 1/4" steel frame. The blank will be 12" wide and 21" long X 4" thick. At 12" wide I would expect to allow 1/8" - 1/4" potential movement. Because it will be inside a substantial frame I may ignore this concern as the frame may not move, but...
Any thoughts?

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4" thick and you want to ignore wood movement? I would not ignore wood movement if it was mounted into 2" thick titanium. Wood expansion can break a weld, don't ignore it. Is that an outside sign? Red oak has very low rot resistance.

one sided or two, hanging I have an oak sign out in front of my farm that I wanted to look aged and it does, I figure 3 years and replace, at this point 4years

Engineer some dados in the wood and put some tabs on the inside of the metal frame, leave 1/8th clearance top and bottom, attach metal in two sections and weld final at corners, let the wood "ride" in the frame.

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If you provide a metal frame that is truly rigid with no give, then the wood fibers will crush when expansion takes place. As the wood moves into the drying cycle, it will pull away from the metal frame.

If you glue or fasten the wood into the frame, the crushing will still occur, and when the shrinking happens, the wood may split.

There are two ways to avoid this, other than building to allow wood movement:
One is to make a plywood type panel, with layers of wood, alternating the grain 90 degrees, or using cross bands in such a way as to restrict wood movement. Plywood and lumbercore are examples of this.
Two is to control the environment that the work is exposed to so that humidity does not change.

I decided to take 1/8" off the bottom edge and 1/16" off the adjacent angled edges. I think this is sufficient to accommodate movement. It will be inside a church, so it will never see dramatic swings in humidity, but better to allow some potential for movement.
Thanks for your replies.

Tim,

Good decision. Jame's plan would be a good one to follow.

However don't think that just because this is in a church it won't be affected by changes in humidity. Only two things can keep this wood from moving......either a climate controlled environment or divine intervention. At least you're in a good place for a chance at latter.

BH Davis

On a pretty sign That Reveal will not be where the Eye travels !!

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If the grain runs left to right, then you will not have any movement in that direction, as wood does not change its size along the grain. You can have about 8% movement in width which is vertical. That is 8% of 10", or about 3/4" wet to dry. A really good waterproof coating will reduce this movement by avoiding the higher MCs. Quartersawn will also greatly reduce movement. A species like white pine will also lower the movement substantially.

Red oak is subject to decay with repeated wetting, so white is longer lasting.

The movement from wet to dry with oak will also create or open cracks on the face. I do believe you are using the wrong species.

Will this be one 4" thick piece of oak? There are very few people that will attempt to dry 3" oak as the cracks are severe.

As long as we are here, it should be mentioned that the Oaks react with the ferrites in steel to make a black stain that could easily discolor the carving, should it find itself in the wet or weather. Good finish and maintenance (on both wood and steel) can prevent or minimize this, but it is very hard to defeat long term.

Nevermind wood movement: how do you get a 4" thick piece of material to fit flush both sides with a 2.75" frame?
I'm sure that the design's already set, but for the future try working with the mechanical properties of your materials. For instance, the resilience of steel can (with the right shape) accommodate the expansion and contraction of wood: no divine intervention required.

Engineer some dados in the wood and put some tabs on the inside of the metal frame, leave 1/8th clearance top and bottom, attach metal in two sections and weld final at corners, let the wood "ride" in the frame.