The Mouse is right...
Brass was much more available in the first half of the 19th century. In the U.S. the adoption and building converters using the Bessemer process allowing better quality and larger quantities of steel pretty much coincided with the arrival of the 1860 Army model. This is one of the advances that allowed the rebated cylinder on the 1860 to be strong enough to work. Before that even the Dragoon size and Walker size cylinders had strength issues.
Most of the "steel" was in reality iron or very low quality steel with many imperfections and very irregular grain structures.
Another reason they used brass is that that could have brass castings made very inexpensively and then finish machine them. Iron castings were much more expensive, full of voids and inclusions and not readily available.
An exception was when Colt's made the back strap on the Army models out of iron at the request of the Army ordnance board who had already requested a larger grip at the Cavalry's request to better work with hands in gauntlets. They actually wanted grips with the girth of the Dragoons and the clearance to the trigger guard. This was awkward because the 1860 frame is actually an 1851 frame with a single difference (the cylinder clearance cut). The offered a lengthened grip (even longer than the Dragoons) to the board and they accepted it. The engineers and mechanics at Colt's had changed the back strap to iron to address a concern that the grip might not be strong enough with the full brass grip frame. This was unfounded and they even offered full brass as a factory option. But so goes the give and take of winning a military contract.
There is a simple reason that the "London" model Colt's had iron grip frames. They were made in England which had a steel industry more advanced than the U.S. It was to the point they could produce iron parts competitive in price with brass because the reality its that iron is more abundant than copper, but it was simply a matter of technological capability.
A lot of people don't realize that the Iron Age came about not because that it was better than the Bronze of the day, but that Tin was the "strategic" material in Europe before the 8th century BC and remained so for the next two thousand years. To most unfamiliar with metallurgy they are surprised to hear that for over a thousand years iron was much weaker than bronze.
It was used because it was plentiful and didn't require tin. Most people also don't realize that "Gun Metal" is not a color, it describes a type of bronze used to make firearms. The reason that Henry rifle receivers were made of Bronze is because it was stronger than the more common brittle irons and most steels. The reason that the material was switched from the original iron receivers on some of the earliest Henry rifles is that it was extremely expensive to buy the low carbon iron that could be machined and have the strength necessary to the very hollowed out receivers.
The world radically changed in the middle of the 19th century with the advent of quality steel and the American steel industry quickly caught up and surpassed most of Europe.
~Mako
