You have some good points. I am thinking about a horse harness for example. After a lot of use the leather darkens, the edges soften. It looks used. I am trying to get mine leather to look like I just bought it in St. Louis. I want it to look a bit like I have been in the mountains for at least a few months.
Ah, OK, now that we know what you want to do, it is much easier and reminds me of something we did in the early 1990's.
I returned from a one year unaccompanied tour on Okinawa to find the wife had moved us off base housing to a housing development in the country that had been originally built for people who could otherwise not have horses, could keep some in their large backyards. She had also joined us to a group of Amateur "Horsey People" as I called them who mainly did horse gymkhana's (weekend rodeos for Juniors) and some Amateur "Show Ring" for those who preferred that.
Anyway, they had a "Secret Santa" Christmas Party not long after I got home and the most popular items by far were two new pair of leather reins, which were "stolen" back and forth many times before the last person drew their number. Of course those reins were machine sewn, with some hard edges and needed to be broken in.
So in the following months, I found out what size and length reins most of them used and hand sewed a new brown set in that size/length for the next Christmas party. However, I "pre- broke them in" after I finished sewing them, so they could be used right away. They didn't look as "Shiny New" as the cheaper machine sewn reins from the Party the year before, so they were not fought over as much. However, I was very glad one family got them for their girl who did gymkhana. They began raving about those reins, just weeks after they first used them and they and others asked me what I had done?
I had done two main things to the leather reins, that you can also do to other horse harness.
I had gently hand skived the top or smooth side of the leather edges to give them a more finished look, rather than the rather sharp edges on the other reins. I also "boned" the sides and particularly the corners where the sides met the rough side of the leather to soften those edges as well. This is a process that goes way back even before the period of this forum. In the old days they Hard Rubbed leather corners with bone or antler or even smooth pieces of wood and sort of crushed down the hard edges to smooth them, though it also looks like they have been in use for a while after you do it. You can do this on much of your horse harness as well.
I had also rubbed the reins across the corner of my bench in a short of "shoe shine" manner after finishing and oiling them, but had placed a clean towel on the bench so as not to stain the new hand sewn reins. This broke down some of the leather fibers and so softened the leather, as would normally have been done after a fair amount of use. I recommend that on a new Harness, before you put it on the Horse's head for the first time.
These are things you can do to leather in a horse Horse Harness to soften the edges and make the leather appear more used, but will also make even new leather look and feel like it is well used and actually make the condition better. The horse will appreciate it as well.
So for the next year's Secret Santa "Horsey People" Party, I made black hand sewn reins and this time, they fought over them tooth and nail, but in a good way of course.
Gus