I think copper is softer than brass. Isn't brass copper and tin? For a tool that you buy for softness, I would think copper would be better. Is there a huge price difference?
Other option of course is a dead blow. I prefer those I think to a solid hammer when I want a controlled impact. Steel faced dead blow probably performs a lot like a copper hammer, only better. (Guessing). I just bought some new Trusty Cooks for carpentry. My favorite dead blows are Snap On hands down.
The term “Bronze” is technically used for any alloy that is a majority copper, with other alloying elements added to increase hardness or corrosion resistance, or other properties.
Most people, however use the term “Bronze” to refer to copper that has been alloyed with tin, since tin was the major alloying element used for copper for centuries.
“Brass” is traditionally used to refer to “Bronze Alloys) that use Zinc, as the alloying element.
Both terms have gotten somewhat confused due to later alloys, that might not be “Tin Bronze” or “Zinc Brass”, but which use “Brass” or “Bronze” as the “Metal Alloy Name”, since the metals have similar properties in one way or another, to traditional Tin/Bronze, or Zinc/Brass alloys.
Appearance wise, Alloys labelled “Bronze” usually have an oranger color, closer to the color of Copper, whereas alloys labelled “Brass’ have a brighter yellower color.
As for “hardness” it really depends on the alloy, and the way the alloy was processed.
Copper, Bronze and Brass can all work harden, becoming harder as the metal gets deformed.
The raw hammers can also cold rolled or hard drawn to help maximize hardness.
Some alloys can also be put thru heat treatments, similar to steel alloys, to maximize hardness.
Generally though, copper will be softer than brass.
Brass will probably be softer than Bronze, but it really depends on the bronze alloy.
The brass hammer would be the least dense of the alloys. Bronze would be less dense than pure or nearly pure copper, but the densities of some bronze alloys are near the density of copper.
If you are unsure of what type of material you need, maybe purchase a split head mallet, and a set of various faces, and then just see what works best as far as hardness, and density.
Garland, Wiha, Thor, etc. all make hammers/mallets, that can use interchangeable faces.
Copper, rawhide, various densities of plastic, etc. are all face options, and I believe the faces are made from standard diameter rounds, so other faces can be made using other materials.