don't know if you can do gum on glass, but i suppose if the plate is subbed properly, you could do it. People do silver, platinum and carbon on glass quite often.
I would bet they are printing the stuff digitally on some material like you can use to make the iron-on T-shirt things from your inkjet, then just sticking them on glass in a deliberately sloppy artsy way.

Quote Originally Posted by Mongo

I'm thinking that, for cost reasons, these must be digital. What's coming off of the corners of the glass is most likely the transparent stock that the image was sprayed onto.

Unfortunately (well, OK, not unfortunately), I never venture into a Starbucks. But there's one near work and I might stumble in there to see if they have something similar and take a close look at it.

Be well.