limestone for impasto

Have you ever tried hydrated white lime. I use it to whitewash the house and stone barns, I can assure you, it glows, and it is a very fine powder product have a try, nothing ventured nothing gained. Don't know what it will be like when mixed with any of the mediums you have mentioned, though.

Hmm.... very bold.... Well - could you use Liquin with it; probably, yes. Liquin is an alkyd resin. If you use any oil, then Linseed would be the best one to use because of its strength. Whether this is a good way to achieve impasto is something else again - but presumably you know what you're doing, and also realize that in the longer term (and not necessarily that much longer either) your painting surface could suffer all sorts of problems, from cracking to simply falling off the canvas (or board in this case; it would be crazy to use such a mix on canvas). I shan't lecture you about that, though - suffice to say that if I were to seek artificial means, as it were, of making impasto, I'd tend to work with acrylics rather than oils - on the grounds that they're more compatible with 'foreign' substances, and once dried and proved, they don't move about on the surface Painters in the past used chalk to thicken paint - either mixed into the paint, or adhered in some way to their support: various other substances have also been used, with greater or lesser (or no long-term) success: the key thing is to ensure they're clean, i.e. not muddy, and inert - so wouldn't or shouldn't change, shrink, expand. When I want impasto in oil, I tend to apply paint, with a painting knife - but I'm not particularly technically adventurous: sounds as if you are! It'll be interesting to see the results you achieve. Liquin should dry more quickly, in short; it's quite a strong substance, not known for cracking; not being an oil, once dried it shouldn't change much. As for limestone - I'd go for the best I could get, and wash it first; always the snag with cheaper products is that you never know what else they might consist of, what impurities they can carry. You'll know, finally, that you can achieve heavy impasto with lead white, if you go easy on the oil.

Acrylics do - generally - dry fast on the support (which is their advantage so far as I'm concerned) but can be kept moist and workable on a stay-wet palette. You can also get Interactive acrylics: I don't use them, but they can be re-activated with water and kept workable with various additives. I can't see how talcum powder would add bulk for impasto - I can see it turning paint to a horrible mess, mind.... It is after all a powder not a bulking agent; if you added it to white you might get a form of, well, gloop - but whether it would form impasto peaks, or settle down into a thick slush which dribbled off the canvas, I don't know: and I'm not going to experiment - paint is too expensive... Whatever it is at base, it's likely to obtain impurities of various kinds - the stuff I've encountered is all perfurmed, and perfume can contain all sorts of chemicals which could interact with the paint. On the whole, oil paint is best not mixed with anything other than a small quantity of oil - and there are thicker oil paints on the market if you'd rather try those. I would never normally recommend any product from the massive Bob Ross empire, but the paint is thick out of the tube....

As well as wax medium as suggested by Alan you could try Calcite (chalk powder) available from Cornelissens and probably other suppliers. I don't use it for impasto but add it to the mix I use for grounds, it thickens and makes the surface more 'thirsty' thereby reducing drying times. For real impasto I use wax and a palette knife.