Grayscale problem



I have a question for when you working in grayscale. My paint is 1/2 black color (fx etac) and 1/2 water. I worked layer over layer and built up my portrait. I want to make the background darker so more black colour. So the question is: what is the best way do to this, keep on building up the layers or reduce the water?
I hope my english is good enough to understand me.
100% correct Nieken. If you can't go darker with your current mix, then use less reducer.
The basic of it all is that your are aplying pigment to a surface. The moment the pigment covers 100% of the surface you will have pure black (when using black ofc :p). By adding more water/reducer to the paint you reduce the amount of pigment you are adding, applying thiner layers of paint. As you will reach complete coverage (100% black) slower this way you'll have more controll with the layering.

When something just needs to be dark you will get there eventualy by adding layer over layer of reduced paint (as you are adding more pigment and are slowly crawling to 100% coverage). If I wanted a background just to be dark though, I'd keep the reducing to a minimum. The reducing is normaly done to gain a bit more control, and with "I just want this dark" that control is not needed and not reducing the paint will save time.
Thank you for the advice, now i know how to finish my work.