I have spoken chiefly on the quality of permanence gained by painting on an absorbent ground, but this is only one among its many advantages. In painting on absorbent grounds the particles of colour remain pure, blending together instead of actually mixing into a tint; and as I have explained in the chapter on colour, you get by this means that great essential for good work-luminous colour. It is also much easier on an absorbent ground to get what is termed quality of colour, and broken tints. Again, your colours remain luminous,always retaining, as we can see in the works of Titian and his contemporaries,their original brilliancy. In painting oil colour over oil colour, on the contrary, the colours in course of time sink together, and however luminous the colour in a picture may be at first, it is apt in time to lose its quality and become muddy. I have seen canvases where a picture,which has originally been painted out, another subject being painted over it, has,after some years,reappeared in parts, owing to the gradual sinking in of the upper layers of colour.,