Revolution in their minds, the children start to march

Black Sabbath - Master of Reality  - 1971

This the album that became quite famous because it is the one in which Iommi down-tuned his guitar quite a bit, Geezer Butler followed suit, and the sound became even heavier than it was before.
And even those of us who aren't extremely well versed in this sort of thing, can notice the difference in sound: this one is not just lower, but it feels so much dirtier that you can almost feel the music stick to your skin like hot tarmac.
While making the sound dirtier, they seemed to make the lyrics, if not cleaner, less "satanic" and with more emphasis in social issues (a trend that had actually started in Paranoid).
I think I still like the first two albums more than this one, although I'm not entirely sure why. I suspect a lot has to do with Ozzy's vocals. I can't tell if he is singing at a slightly higher pitch here, or if the down-tuning of the instruments makes his always high pitched voice stand out more than before.
It also seems much less relentless than the previous two albums, with a couple of short, mellow interludes, and an actual ballad, "Solitude", which is a really beautiful song, but is as far removed from the rest of the album as it can get.
Still, this is a great album because the change in sound was innovative and gives the album a (for the time) unique feel. Also, the best tracks are really amazing.

• The move away from the "satanic" vibe in the first albums is so radical that "After Forever" was covered by an all-out Christian rock band such as Stryper.
• As absurd as it may sound, especially considering the heavy sound of this album, the beginning of "Children of the Grave" is extremely similar to the beginning of..... "Call me" by Blondie. I'm fine with it, because I love both songs.  

Where I'm going I just don't know

John Mayall - Thru the Years - 1971

As seems to be the case with most things that are almost purely blues, I don't know much at all about John Mayall. 

Of course I know him by reputation, but that's just about it, so most of this album has been unkown territory for me.

It includes mostly previously unreleased and B-sides from the 1960s. Which is interesting, because some of the tracks, like "Mama, Talk to your Daughter", and "Crawling up a Hill" sound much older, both in style as in production sound. Which I don't think is a bad thing at all, and I quite enjoyed them.

There's also a bunch of songs from the time in which he had Peter Green as a guitarist, around 1966-67, and these have a much cleaner sound, and some of the guitar work is pretty amazing. And if you're thinking I really don't know much about Peter Green, you're right. 

All in all, I enjoyed this one quite a lot, and while I don't really know how representative it is for Mayall's career, that currently spans over 50 years, this was a very good introduction to his music.

And also....

Hairy Chapter - Can't get through / Eyes - 1971

This band is so obscure that it doesn't even have a Wikipedia entry.

Well, actually it does, in the German Wikipedia, and even that is just a bit more than a stub. So I really don't know anything about them.

This is an album, two albums actually, that I'm pretty sure I will not listen to too much after today. It's not bad musically, I guess, although it doesn't add much to anything. 

The two main problems are: the production is quite bad, and the vocals are dreadful. And if I can feel annoyed by vocals with great bands such as Black Sabbath or The Doors, I'm not too willing to put up with them with this. 

At their best, they sound like just a run of the mill prog/hard rock band from the early 1970s. At their worst, they sound like a bad demo from a bad garage punk band.