Slate.
There are many types of dampcourse in use, the oldest form being pieces of slate embedded into the bed joint. Slate seems to keep the moisture out quite well.

Bitumen.
Another popular method was to mix some kind of bitumen with the mortar, I have seen a lot of this type of dampcourse that has failed due to age and the bricks above have crumbled.

Bitumen on a roll.
Comes in various sized rolls and goes by lots of different names, it is reinforced with fabric. It seems to hold the moisture out well.

Plastic.
The modern stuff, also comes in various width rolls, also seems to be doing the job of keeping the moisture out.

Chemicals.
There was a lot of water proof addatives used in mortar some years ago, most that I have seen have failed, used mainly around the 1950's to the 1970's.
Can be black in colour or clear.

Dampcourse
or DPC (Damp Proof Course

Brickwork.net.au

Dampcourse Failure.
When a dampcourse fails the wall usually show a white fluffy efflorescence on the surface and bricks will fail and crumble.
Musty smells in rooms usually accompany a failed damp course.

Also see, Brickwork Faults.

Bitumen

Slate

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