Acrylic ink with masking fluid
The pictures described as in acrylic ink and masking fluid are done by alternately painting in acrylic ink, and, when this has dried, painting over that with masking fluid and then painting in more acrylic ink. This allows something in the foreground to be painted first, and protected by the masking fluid as an object behind it is painted. Towards the end, the masking fluid (now dry) is rubbed off. It is only at that stage that the painting finally appears which means lots of thinking ahead and imagination is needed to guess how it will work out (and sometimes colours need to be adjusted). To illustrate this, here are two paintings, both followed by steps along the way:
My 2015 Christmas card (below) took this a little further, using a new technique (to me), of wetting an area of paper and flicking acrylic ink at it from a stiff brush, so that the ink landing in the water spreads, and the ink landing elsewhere forms small spots which contribute to darkening the background. In the end, the actual printing came out much darker than what was sent off, but worked rather well.