Beware the dangerous (and useless) 'Screwdriver' test!
This is NOT a realistic or accurate test of the performance of a mortar!
We've had contractors comment that "... the tile come up easily ..." or "... the tile came up clean ..."
I REPEAT: This is NOT an accurate test of a mortars performance. The fact that the tile come up only shows the massive force that can be exerted with a "lever" You may remember from high school physics that Archimedes said " give me a long enough lever and a place to stand and I'll move the Earth!" And he was RIGHT!
The fact that the tile came up "clean" only indicates that the bond of the mortar to the subsurface and the INTERNAL strength of the mortar was stronger than the bond of the mortar to the tile. This in NOT necessarily bad! Remember that if you take a 5,000# strength chain and pull with 6,000# pounds of force the chain will break at one of the links. This DOES NOT mean that the broken link was weak or "bad"! It only means that it broke after its strength was exceeded!