Well maybe to reduce pitting. I said undercutting because I believe
sodium silicate is used when etching to reduce undercutting. I have no
idea of why it does that, but if it tends to reduce the etching action
where the etching is beneath the resist, I think it would also reduce
the etching action is pits.

I will do a little googling and see what I can find. I think the name
used by etchers is dragons blood, but am not sure.

Dan