Black glass means the photo has been taken on black glass instead of film as a medium.
The black glass has been treated with a layer of Collodion which serves as a holder for the silver nitrate which gets applied after the Collodion layer to make it light sensitive. The plate is then being exposed and immediately developed and fixed.
Wow, thank you all for your wonderful comments! I find it very expressive and that suits my brother pretty well so I'm very happy with it. I hope to do more with him like this.