John Britt flocculation/deflocculation demo 

You added too much.

If you add too much it thickens. You only need a few drops of sodium silicate.

I added as much as always and it thickened also. I add only liquid glass to my slips and there was never a problem, after a year and a half of work with it, it started getting thicker. What am I doing wrong?

It's a deflocculant. It makes the particles of clay separate and flow more liquidly without adding water.

Add Epsom salt dissolved in warm water. He does this in the video.

sounded like "nephsite", which could be nepheline syenite

Neph sye = nepheline syenite

Sodium silicate AKA water glass was added to the clay to deflocculate (thin) it. Epson salt was mixed with water, and used to flocculate (thicken) the clay.