PloughGuy

I am looking for a way to find mortar joints behind plaster. This is kind of the opposite of a stud finder. In my 150 year old house the mortar is lime and sand, so the walls are held together by hope and gravity rather than glue. The plaster is troweled set plaster that sort of sticks to the bricks. Drilling into a joint when trying to hang a picture can result into a variety of unsavoury results. I want to find either the centre or the edges of the bricks. Since the mortar was mixed by hand, the mortar lines are sometimes very thin (Right, chaps, we need to get another course done before knock-off time, and Clive is too knackered to mix another batch, so make it last! I think the real reason is almost the same - the lime mortar was mixed some time in advance to give it time to do what it had to do to become mortar, so if they ran out because someone tipped over a wheelbarrowful, they had to make do with what they had.). In this case, the tool might find what looks like a very large brick. No problem - I can drill my hole 1/4 in from the edge to get a brick centre. Stud finders are tuned to find wood or metal through plaster. My question is - has anyone successfully used one to find mortar joints through plaster? Thanks for your help.