Another version of this scene is done in Venetian Bead Stringers:

Venetian Bead Stringers
1880/1882

Notes:

Exhibitions

Gondola Days: Isabella Stewart Gardner and the Palazzo Barbaro Circle, 2004

• See the year in review  1880
• Venetian Studies
• See Overview of Venice
• Essay Street in Venice

Is it known where [this] very interesting interior was/is located? 

I first thought of Palazzo Pisani-Moretta due to the very unique stairway in the background, but the room's proportions do not fit. As I'm quite interested in everything that has to do with venetian palazzi, I'd appreciate any hint on this wonderful picture. There are two possibilities I can actually think of, but I do not know them in reality (source: Bassi, Palazzi di Venezia, Venezia 1976):  Palazzo Sagredo (not probable due to lack of portego decorations), Palazzo Moro Lin (more probable). Or has this painting to be interpreted as a "capriccio" ?
 
Sincerly,
Jan-Christoph Rößler
www.jc-r.net 
 
From: Natasha

That's a great question but one that I don't know if  we have an answer to. The central hall of these palazzi, as you know, are called "portego" and run the length of the building from front to back. In the painting above, we are looking towards the "land side" -- or the back side of the Palazzo -- the front being open onto the Grand Canal. These portego often had stone floors and in the 1880's a number of these once beautiful Palazzi were in poor repair, some used in the industry of making glass beads etc.

One of the best accounts that tries to tackle this subject was in Linda Ayres chapter called "Sargent in Venice" which appeared in "John Singer Sargent", Whitney Museum of American Art, edited by Patricia Hills, 1986; pp. 49-73

She writes:
 

Although it is tempting to think that the interiors depict the Palazzo Rezzonico, where both Whistler and Sargent had studios at different times, visitors to the Rezzonico agree that its proportions are grander than those depicted in Sargent's and Whistler's pictures

Memoranda in the files of the Stirling and Francine Clark Art Institute and the National Gallery of Art. Douglas Lewis of the National Gallery has suggested the early Renaissance Palazzo Corner-Spinellie as a likely candidate