Historic New Orleans Roofing Slate w/ painting

Writing on the back of this states " Historic New Orleans roofing slate.  These slates were mined in Pennsylvania and floated down the Ohio and Mississippi Rivers on rafts.  They were used for roofing material after the disatrous fire which occurred on Good Friday, March 21, 1788.  each one was individually hand shaped.  This one came fron an old building in New Orleans.  Archie's Sketches, 4804 Elmwood Parkway, metairie, Louisiana 70003"    

Underneath this is another paper reading " Royal Street, New Orleans.  A visit to New Orleans is not complete without a stroll down Royal Street, or Rue Royale, as it was named when the city was a jewel in King Louis' empire. Today, famous restaurants, stores with the finest in antiques, and quaint gift shops greet you.  Among the many historic buildings is the La Branche, or better known as the "Lace building", a faavorite of the artist and the photographer.  It was built in 1835 for Jean Baptiste La Branche, a wealthy sugar planter.  Directly across from it is Sieur george's House built in 1811, the first three story structure in the French Quarter.  Brannan's Restaurant building at 417 Royal was built in 1801 for the maternal grandfather of the French impression painter Edgar Degas.

The artist, Knut "Ken" Engelhardt, was born in Germany and educated in both German and French art schools.  He won three gold medals in the olympic of Fine Arts in Paris in 1964 prior to coming to the United States.  He has exhibited his paintings in many cities in Euroope and the U.S. in addition to his work on canvas, he has perfected painting on the historic roofing slates salvaged from the old building in the Vieux Carre' - using the weathered patina as the background.

This piece came from an auction box lot and I would like to find out more about it, including it's value. Any info would be greatly appriciated.  Thank you