@kirbyklekter - Kevin's explained it pretty well. Though, to be honest, I don't think I've ever seen a dry lubricated sintered bronze bearing. I know they exist, but I think for most common applications like vac motors, they're gonna be just oil lubricated ones. I dunno, maybe newer designs are moving over to dry?
Anyhow, I personally would have trouble telling a sintered bronze bearing from a solid brass bushing / bearing, by color. Especially given that they're usually dirty after being in service. If you clean it off, you can actually tell the sintered ones are, well, sintered. They have a unique textured color / appearance, as opposed to a solid brass metal, which would have a uniform color.
The real giveaway is the lubrication method. A solid bearing has no way of getting oil to the spinning shaft on its own. So, it would need a method of getting oil directly to the shaft, like an oil wick or ring oiler that touches the shaft directly. Which requires a hole in the bearing. This is generally the case for very old motors, and for not so old, but heavier duty motors. For example, my Eureka model 10 (~1927) has solid bearings, thus it has holes drilled through the bearings, and a oil cup underneath each with a wick that go up through the hole to touch the shaft.
Whereas a sintered bearing is effectively a metal sponge, so it has its own oil IN the metal. These will (hopefully) have an oil wick or - more commonly - felt to hold some extra oil, but that will NOT directly touch the shaft, only touch the outside of the bearing. That's the giveaway - whether the oil wick or felt touches the shaft or not. It's often the case with cheap, small C-frame motors that they have no oil felt at all. Now, I suppose some of those would hopefully be dry lubricated, but I think the reality is that they're built so cheaply, that the oil in the bearing is considered good enough for the 'lifetime' of the motor - which will be like 5 years. Anyway, as the oil is used and pulled out of the bearing, the extra oil in the felt will get capillary-action-ed into the bearing. But see, that can only work if there's still oil in the bearing. Once it's run dry, it can't take new oil on its own.
Fortunately, you can put the bearing in oil, in a vacuum chamber, and suck the air out. I've forcefully re-oiled quite a few, it seems to take several hours to get the air out if they're dry. (I don't really know how effective massaging oil into one would be, then, but hey.) Anyway it's a piece of cake if you have a vacuum pump. You just set it and let it run all day.
As a couple of examples, my Cadillac vacuum (1938) has a ring of felt around the bearings to hold oil, with oilers that feed those felts. The machine was WELL used and beaten up, and had bone dry bearing felts, so that was a pretty good indicator that the bearings were run dry. Sure enough, they bubbled all day in a vacuum chamber. On the other hand, one of my handheld Cadillacs (~1950s) was in great shape, and looked well cared-for. When I opened it, the oil felts were still quite wet and the shaft still had a very slight slick of oil on it. Someone actually oiled that machine, amazingly.
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Kevin - I'm curious what chemical you use to clean a dry bearing, to prepare it to be made wet.
