To remind you: this is not a lesson! BUT ...
... if this system was good enough for Diego Velázquez, then it's good enough for me (although I got more hits on my blog than he ever did! ........... just saying.)
Today I added grisaille planes (which the dictionary defines as: decoration in tones of a single grey [gray] designed to produce a three-dimensional effect.)
I mixed the grey (gray) using a W & N Carbon Black (a slow drier) with a W & N Alkyd Titanium White (a quick drier) to speed up the drying process.
I made a grey (gray) string mixing proportions that gave me a match with each value on the scale shown above (right).
I placed splotches of the mixes on a throw away paper palette, with pure unmixed black on the extreme right and similarly unmixed white on the extreme left.
I them mixed a medium for this layer of painting thus:
Using the value finder to identify the values on my reference photograph, I replicated the value planes on the painting, observing not to blend at all; just laying in planes of grey (gray)
Finally, I just softened the lines where the grey planes met, with a dry and paint-free soft sable brush - avoiding fully blending
I've left it to dry overnight.