How to remove hardened adhesive between glass mosaic tiles

[Dunkgrease]

In our old Portuguese property the shower floor is tiled with white glass mosaic tiles. When they were laid (years ago) a lot of the adhesive which is very hard squeezed out through the gaps. The tiler never bothered to scrape out and just grouted as they were. The result is that there are unsightly grey patches of adhesive interspersed with white grout.

What is the best way to get this old adhesive out so that I can regrout. Mechanical means or is there some form of acid wash?

Thanks

 

[Time's Ran Out]

Thin blade on a grinder or one of them multi tools.

 

[demi34]

Little drivel machine with 35 mm blade

 

[demi34]

Sorry drimel machine

 

[Slippery]

Diamond blade multitool