Kindly tell me whether there is any harm in putting a _thin_ coat of mastic, softened perhaps with a drop or two of oil, over works _finished quite recently_ but _begun_ a year or more ago? If I understand rightly, cracking is caused by atmospheric action through the _back_ of the canvas, by _distension_ of underlying partially soft paint and, consequent disruption of the upper, harder layer of varnish. If the first painting is a year old, is it not tough enough to resist the atmosphere, and is it not _anyhow_ pretty safe when the canvas is _backed_?
I suppose "Mutrie yellow" is quite safe alone and mixed with other pigments?
Thanks for your note. Yes, I do like the white oil, but I add copal to it if I want it to be very drying, or mix copal on the palette with a slow-drying colour, say a lake. This, I suppose, is all right; if so, don't trouble to acknowledge this. The oil of orange is delightful on account of its smell, but dries less quickly than turpentine (rectfd. spirit). Is it not _always_ better to have _some_ resin in a picture _throughout_ since it has to be varnished at the end?
I am so much enamoured with the method, so far as vehicle is concerned, which I have used during the last year, that I should like to feel quite certain that it is _absolutely safe_. I use a "single-primed" canvas, and underpaint with "Bell's medium" and rect. spir. turps., which, under your advice, I have in _small_ bottles, so that using it freely a bottle lasts a very short time, and the stuff is therefore always fresh. The mixture I _use up to the end_ (except when I now and then use the pigment _alone_), and letting the turps. rather _preponderate_ as I advance. I have found to my amazement that this mixture dries even in winter weather excellently, and that I can use with it even scarlet madder and aureolin, which, at least the former, hitherto I never attempted to use except stiffened with amber or copal; and I further find that this mixture, though of course it "sinks" to some extent (and especially with the blues), in the main bears up very fairly, incomparably better than I should have expected, and in fact quite enough. Before beginning to paint I rub over the part each time with Bell's medium and saliva nearly equal parts, or say five oil to four saliva beaten up with the knife on the palette to a white mucilage. This, if left alone, makes a good varnish, and is delightful to paint into. So far, so good; at least I suppose so. (Do you see any elements of danger? cracking? darkening?) But at the end something must go over it all, if only to lock it up (I suppose), certainly to get uniform gloss and strength. I propose in the Academy to put Roberson's medium over the whole of my large one and to retouch with the same. A portrait on to which I _don't_ intend to work I should cover with mastic and _a little poppy oil_; there is no harm in this, I suppose, and the small quantity of mastic is not likely to yellow, is it? I know this mixture _won't come off_, but why should it?