There is a ghost economy at work in Old City.  Hand-painted advertisements from a different era, faded to the point of hardly being there at all, emerge like apparitions during building renovations.

"The signs have been buried behind a brick wall, or a building, or stucco for however many decades, and all of a sudden it's there again," said Lawrence O'Toole, author of the photography book "Fading Ads of Philadelphia." Sometimes it gets covered right back up and it's only there for a couple weeks, or less."

O'Toole started taking photos of old advertisements painted on brick buildings as a thesis project 20 years ago, when he was a student at Drexel.

He was fascinated by the lettering and layout of the old signs, and how time had its way with them. That project turned into a blog, The Ghost Sign Project, and later, the book.

O'Toole will present his photos at the Philadelphia History Museum at Atwater Kent on Thursday, May 16 at 5 p.m. and talk about where they came from and why they have held his interest for two decades.

He will also talk about where, exactly, the images were taken. As neighborhoods evolve, and their demographics change, and fortunes rise or fall, the advertisements give voice to the remaining red brick, bearing witness to what had been.

It's not always easy to figure out. Many of the ads are faded and illegible. O'Toole, now a design manager in Manhattan, spends his spare time poring over records to find glimpses of the ads, one historic photo at a time.

"It's less of an academic exercise and more visual," O'Toole said. "I've been doing a lot of visual searching through photo archives. A lot of times, that's the only way to get to the point of what these things said."