On Dec 9, 5:27 am, Ben wrote:
I recently noticed some cold air around the bottom of our fireplace
hearth, which is brick and mortar. A quick look showed that the
mortar under the bottom brick has worn away, in many places quite
deeply. Should I fill with mortar, or could I use some other sort of
sealant (foam?) to fill this space? If the former, any tips for
someone who has never dealt with bricks and mortar?

Thanks!

-Ben

I think you are probably a little out of your depth on this project.
If this gap is between two items of masonry then you need to use
mortar.
If this gap is subject to the fire itself you will need to use
fireclay based mortar.
If this gap is between the hearth and the surrounding floor you may
have other options to provide some flexability as the two different
materials move at different rates.