I was so intrigued by the last Blog post that I made up this simple swatch of six colour boxes.
I have used Vallejo Game Colour Bloody Red (72010) painted on to artists sketch paper. I made sure that I shook the paint bottle prior to conducting this experiment.
I have started with the 100% Bloody Red, then added talc in roughly 10% increments.
I used Johnson's Baby Powder - a perfumed Talc.
Vallejo Game Colour has very good coverage.
Vallejo Game colour does dry with a very slight sheen.
Adding talc does effect the colour, best seen in the 40% box.
At 10% there does not seem to be any change in the colour, but I would suggest that the colour is a little more matt.
The opacity seems to have been affected, with 30% and 40% affected most.
By adding 50% talc, the paint became much thicker and too thick to be used for painting miniatures.
I needed to add additional water to the 30%, 40% and 50% boxes as the paint was too thick to use properly.
I can see a case for adding a small amount of talc to acrylic paints. I would suggest that 5% - 15% maximum.
I would like to conduct further tests - possibly over a Black undercoat to see if the paint performs differently or has different opacity values.
I hope that this helps.
Updated 13.36 02/05/2014
This post from Vexillia gives more details.