I’ve had a problem where an outside door swelled in hot and wet weather so that it did not quite fit in its frame, and repeated impact has knocked out a section of the pointing around the frame. I assumed I would just have to fill in the gap with mortar, but the gap behind (only a couple of inches deep) was actually filled with some dark stone material, and this is what has crumbled. The mortar just fell off because of this.
As it’s just a gap between the frame and the bricks I could conceivably fill it all with mortar, but does anyone have any idea what might have been in there? The house is over 100 years old but it’s not clear when this door frame was fitted, or even whether they did a good job of it.
Does anyone have any thoughts on what I should put in this cavity?