Help with problem underpainting darks
 

I'm doing an underpainting with verdaccio and when it dries, certain dark parts appear to have become lighter. For example, I painted the pupils of a portrait with black and the next day it looked like I had painted a blind person with no pupils. This lighter value seems to regain its original value after I oil it or spray it with retouch varnish.

Can somebody explain to me what happened and how to avoid this problem?

Thank you

Dear Tito
 

Dear Tito,

Try using a black that you mix using red, green and blue or use Ivory Black. This is what painters have used with success in the past. Hope that it works for you. Good luck. :)

This is a good page for some information:
http://www.pixation.com/art_tips.htm

Thank you Celeste for your reply. The dark I have been using is a combination of Mars Black with a little bit of Chromium Green Oxide. However I believe that the problem is not for using the wrong kind of black, but it has to do with either bad reflection of light or a sunken area of paint, since the problem does not occur everywhere in the painting, only in certain spots.

Sounds like...
 

Sounds to me like a classic case of thinners. Are you thinning? Thinning paints without oils is wise for underpainting-it will however leave those darks to dry flat. This really ought not bother you. It is only underpainting.

A classic case of sinking in. I would spray my verdaccio with retouch varnish each day before beginning work. This brings the values back to true.

Thank you Tim, thank you Michael.

You're both right, I'm not using oil for the underpainting and therefore the paint dries flat. Retouch varnish or medium seem to bring back the original value. I suppose retouch varnish is preferable over medium in order to follow the "fat over lean" principle. However, when I spray retouch varnish, the varnish layer looks spotty and if I try to make it even, then it runs with the paint because it's too much.

Yep
 

Uncle Michael is right and retouch will give you a good bond to follow-up with. But, If you can just think of this as temporary UNDERPAINTING and move on you'll be fine. It will all be just a whisper (if even that) later. Try to focus on the drawing and edges etc. Safe your stress for later.

I used to use retouch varnish until an artist trained in Majorca showed me an alternative: A jam jar with one third oil, one third thinners and one third water, shaken vigorously and then the resulting emulsion thinly brushed in to the dry painting. This brings back the wet tone without adding another layer.

I have found it a lot easier to manage than retouching varnish. Does anyone else use this?

Thank you, Tom Wise, for the interesting formula of blending one third oil, one third thinner and one third water to deal with dry sunken spots of paint. It sounds to me like watering some common medium. I'll give it a try.

I find very interesting painting with verdaccio, because it looks like a charcoal drawing.

As soon as I can get a decent photograph of this underpainting I will scan it to get further advice regarding the glazing part.

Water
 

What's the water for? Aren't you talking about oil paints? Humidity (water) can cause all sorts of trouble with adhesion with subsequent coats of varnish, oils and paint. Parrish would place his paintings in the hot sun to cook off the moisture from the oil's surface before varnishing.