I need to put a slight curve in a horn butt plate. Plate is 120+ years in age. Plate fits at toe and heel but is a bit high in the center, about the thickness of a business card. I want to put a very slight bend in the plate between the toe and heel. How do I best get this accomplished without chance of cracking?

Maybe heat the plate in water until it flexes. Then screw in the plate and attach a strong rubber band to the trigger guard and over the plate at the gap...Geo

If you heat the plate gently( hot water, hair dryer), you should be able to put a small bend, down, in the middle ( block up the ends with small rod, and put weight or padded clamp in middle). Once you have this bend and the plate cools, the screws should pull the ends down, making a tight fit.
Mike

I have read (on this forum) that a short soak in neatsfoot compound will soften the horn somewhat. Would this be advised before applying the heat? Or, after 120+ years likely to be ineffective?

LetFly,
I didn't see that posting, so I can't say it won't work, there are a lot of people that know more than I. It's your plate, do what you think is best, it may crack regardless what you do.
Mike

I've always put the horn plates in boiling water.
Let it boil till bendable & pliable.

Quickly set them it place on the stock and you can press it into shape with simple hand pressure. But the hot matr'l isn't too pleasent to handle so some weight is the usual method to push the plate into it's new shape.

A partially filled shot bag works well on the face of the plate. It won't take a whole lot of weight if you get it on there while the horn is pliable.
It cools and stays in the new shape.

When the curve is so slight that you are trying to produce in the center of the plate,,you sometimes are farther ahead to slightly,,very slightly shim the ends of the stock face to produce a slightly exaggerated curve to the plate.
The same thickness of the business card that the plate needs as a curve in the center can be built up at both toe and heel. Any hard edge must be shaved or sanded off of the card or that will show when the soft horn is pressed into it.
The whole idea is to create a slightly deeper curve in the center by a few .000 and then the attachment screws will draw the plate down nice and tight.

Over kill perhaps,,but it assures that there is no gap when it's all done under the plate at the mid section.

If you use clamps and rubber bands and that sort of thing to pull it into shape, the smaller area contact pressure points of those things can often imprint into the rear face
of the horn where they contact and that leaves their mark when cooled as well.
If you use clamps or bands, spread their holding power over the area with something like felt or leather pads so you don't leave an imprint.

If, for some reason, you are afraid that the plate can't be bent without cracking or breaking, there is another option. A little more work, but an option just the same. Put the plate on the gun with the screws, but leave the screws slack enough so that you can slip a long strip of coarse emery cloth between the plate and the wood, grit side down towards the wood. Holding the end of the emery strip with one hand, apply gentle pressure down on the middle of the buttplate with the other hand, and slowly pull the emery cloth strip out, allowing it to cut the wood as it comes out. You pull slightly up ("up", meaning that the buttstock is in a vise with the head down and the buttplate up) as the emery is pulled out. This slight upward pull prevents the cutting side from rounding off the edge of the butt.

This is repeated many times, until the places that need "relieving" have been cut away by the emery. The emery strip, having no cutting action on the top side does no damage to the back of the buttplate whatsoever. Done with patience, this method will result in as near a perfect fit as is humanly possible, with a very large percentage of contact between the two surfaces. This is the method I used to use when fitting brass or iron crescent buttplates to rifle stocks. I've used it on shotguns many times as well.

Don't take my mention of this as an effort to try to prevent you from bending the horn buttplate. Only you are able to look at it and decide it you are willing to go the route Kutter described. I only mention it as a possible alternative, that will require no refinishing of the wood or the buttplate, if done properly.

Best wishes with it!