Let me start out by saying I hate cold lather! When I bring the shaving brush to my face for the 2nd, 3rd, or 4th passes, cold is not a quality I'm looking for. Like many here, I start out with hot water in my bowl, but alas, it just doesn't last long enough to keep my lather from cooling off before I'm done.
I've read cautions about using mug warmers as you can damage the hair on the brush if it becomes too hot. Wandering around Wally World the other day, waiting for my wife, I noticed a rack of mug warmers. It had a relatively small heating surface which was surrounded by a quarter inch metal lip. Believing my mixing bowl to be larger than the heating surface, I dismissed it. As I continued waiting, and waiting, I began to wonder if it might not be a blessing that the bowl was larger than the heating pad! My bowl should rest on the elevated rim which would keep my bowl a quarter inch or so above the direct heat. I decided to take the plunge with a significant investment of $2.50! What the heck, it's Christmas!
When I got it home, sure enough my bowl rests securly on top of the rim just as I'd hoped. The next morning, I plugged it in 2 or 3 minutes before I started my prep and soon it was ready to go. It has a high and a low setting with low being 110 degrees and high, 120 degrees. Neither temperature would likely do harm the brush but no sense taking chances and I'm just looking for warm, not hot. After my first pass when the brush touched my skin.....ah, warmth! I can even remove the bowl and hold it in my hand with the bottom of the bowl resting in my palm comfortably. Just the right amount of heat! No drying out of the lather was an additional bonus since there in no intense heat.
Here's a picture of the unit itself.
And with mug sitting atop the unit.
Gotta love the feel of warm wet badger hair in the morning!