Alizarin crimson

Yes, the Alizarin Crimson pigment is not compatible with acrylics, and so an alternative pigment is used. And no, Alizarin crimson hue, in acrylic, is not the same as the pigment of the same name in oil- you need to took at the pigment letter and number to identify it:  PV 19 is a good colour in acrylic (and oil and watercolour) and it can be darkened with, eg, Pthalo Green (carefully - Pthalo Green has a nasty habit of swamping anything with which it is mixed). I'm a sleepy old soul this evening, and will come back to this tomorrow if I've anything useful to add - in the meantime: in oil, Alizarin Crimson is fugitive in tints (i.e. mixed with white) and in glazes; PV 19 is much more reliable; even Rose Madder Genuine is marginally preferable.  Though ironically, Alizarin Crimson was meant to be a more permanent alternative; and isn't.   Crimsons have always been problematic pigments, unfortunately - and "Permanent Alizarin Crimson" does not,sadly, mean "permanent" in the sense in which most of us would mean the word: it depends entirely on the pigments used to make it: look for PV 19: that's a reliable crimson/violet pigment which is available in differing strengths, and we can adjust it ourselves.  It's not, though, a direct alternative to the original Alizarin Crimson, with its blue-ish undertone; there is no entirely reliable alternative for that, as yet.  

Avec mon plaisir.... Benzimidazolone crimson is said to be lightfast, and I probably HAVE used it, but I'd need to tip my crowded paint box upside down to see if I've used it in oil: the acrylics are easy to access (fewer of them) so hang on a sec .... fumble fumble.... ah.  That's interesting.....  My standard Acrylic brand is Cryla (because I've found it the most reliable).  I have two tubes of Alizarin Crimson (hue) one older than the other: it's a mixed hue - one tube reads "Quinacridone PV19, and Quinacridone Yellow Red PR209"; the other tube just reads - eye's  playing tricks!  It's identical!  Not Benzimadazalone crimson in either case. I tend not to use Alizarin Crimson in any form in oil, so my tube - languishing at the bottom of the box - will be PR83; so I won't have used Benzo-etc in oil; I've replaced it with PV19, available as Permanent Rose, Magenta, Rose, Quinacridone violet, and Lord alone knows how many others.  I would use it, for its lightfastness - and transparency.  Would, haven't, I think. I would be pretty sure it won't replicate the qualities of Alizarin Crimson; won't in other words be just the same - unlikely to have that blue undertone which makes Alizarin so useful in portraits.  But there's no real way around that - my PV19 is transparent, is lightfast, is a beautiful colour which can be mixed into all sorts of crimson, dark red, violet hues, both by manufacturers and by ourselves.  I'm pretty confident the same is more or less true of Benzimidazolone (God, I wish they'd think of a simpler name!) - I'll look it up a bit more thoroughly in the days to come, and will pass on anything I can discover. Without any doubt, it'll be more lightfast than Alizarin, or the gorgeous but sadly semi-fugitive Rose Madder Genuine. There are many painters who still use PR83 - I can see why, but I think they're making a mistake.  I tested it in watercolour - strip of painted paper in the window: it was OK for a few weeks, then, quite quickly wasn't - part of it turned brown, the rest just disappeared.   I hope that's of some help, even though I don't THINK I've use the Benzo-etc paint myself.  

This might also be helpful and is certainly interesting; it's from the Natural Pigments website, who are based on the Rublev paint brand.   Notes About Lightfastness Although assigned to lightfastness category III (ASTM D 4302), alizarin crimson may be a permanent color under certain conditions. Franz Hals was well aware of the correct way to use fugitive lakes to produce vivid flesh tints. The ruddy nose and rosy cheeks portrayed by him, which after a lapse of centuries, have shown that this color, when intelligently used, is stable. According to Maximilian Toch (Toch, 1911), if madder lake (the natural lake pigment related to alizarin) is used as a glaze over a color that has been allowed to dry thoroughly, it will remain permanent. Still, if mixed with any mineral pigment, such as ocher, lead, sienna, etc., it is altered and loses color. This Toch based on his observation of Rembrandt's use of color in works like the Anatomy Lesson, in which 'upon close examination, the flesh tones have suffered very likely due to the fact that the lake used in glazing was mixed with the under-coat.' Based on this observation, Toch makes this recommendation for alizarin reds: It is, therefore, reasonable to assume that alizarin or madder lakes should be used as glazing colors over a properly dried surface. Alizarin or madder lakes will not decompose when mixed with various blacks such as bone black, lamp black, and carbon black. Madder lake may be mixed with oxide of iron that has been burnt, but may not be mixed with any raw iron color. For instance, madder or alizarin may be mixed with Indian red, forming a color known as Tuscan red, which is perfectly permanent. It may also be mixed with burnt sienna, burnt ocher, burnt umber, etc., but is fugitive when mixed with raw ochre, raw sienna or raw umber. The chemical colors like flake white (white lead), zinc oxide, chrome yellow, Naples yellow, and chrome greens all bleach it, but colors like quick silver, vermilion, cadmium yellow, and all of the blacks do not affect it. The safest way to use it is as a glaze over a thoroughly dry ground. Madder lake deepens considerably when placed in a dark place, but is revived when subjected to bright sunlight. Vermeer more than once placed a glaze of lake pigment (carmine lake) over vermilion. He first modeled the object to be represented in various tones of vermilion, using white to lighten the tone and black to darken them. Once the area was dry, he glazed over it with a red lake. The recommendation of raw natural iron oxide pigments and the mention of bleaching with "chemical colors" requires further investigation because other authors have made contrary claims. Maximilian Toch, Materials for Permanent Painting, New York: D. Van Nostrand Company, 1911. pp. 85–87. So - me again: there you are.  To summarize, PR83 should be safe used as a glaze over dried colour, but don't use it in tints (i.e. mixes with white, including the yellow ochres and Naples Yellows).  This has been a very interesting trawl, and has reminded me of things I've forgotten.   To be entirely safe, and as you're happy with the colour, I think you'll be fine using your Benzo-yammeryammer crimson/red.  But - you do have a choice, if you were thinking of glazing rather than mixing.  

Well, I've woken up to thick fog - we're all hanging on here for the first signs of Spring; but I wouldn't object to a bit of snow first - I'd like to paint it again. Almost without exception, acrylic colours are lightfast: for one thing, they've all been developed over the last 70 years - with an eye to achieving permanence - and for another they don't have the oil binder, which changes over time.  For that matter, neither does watercolour, which, as Rowland Hilder remarked in one of his books, can fade alarmingly: "one thing that really bugs me", he said (he was born in the USA: not usual language for an English gent of his vintage) was having to view watercolour exhibitions in a darkened room; he'll have meant the older watercolours, because big strides have been made in improving w/c lightfastness - except for the madder lakes, including PR83: I'd never use them in watercolour. Now, say that and someone pops up to say "but Ron Ranson did!": yes, he did.  But then he didn't have anything else at the time, other than Crimson Lake, which was no better, and anyway a rather duller colour.  The advent of the quinacridone, perylene and pyrrole pigments changed everything for watercolourists, and I bet Ron would have leapt on them if he'd had the chance.  I do wish though that we could have names for the newer colours which sound less chemical - benzomidazilone, Azo, pyrrole....  It DOES tell us what's actually in them, though - whereas Fairycake Pink (I just made that up, don't go looking for it) tells us nothing; and there are a lot of colours like that on the market: Potter's Pink, Hooker's Green, Monastral Blue (pthalo, to you and me) - if you know you know; if you don't, those names won't tell you what to expect, still less what they're made from.   But then, does it help to know that a paint is based on Benzowotsit?