Quinacridone gold, also known as PO49, Pigment Orange 49, was a golden pigment used in the automotive and artist’s paint industries until 2001, when production reportedly stopped. The pigment had gathered somewhat of a following within the artists’ community. Artists such as Jane Blundell explored alternatives on the market, and others like Sandrine Maugy bemoaned the last supply of pigment having been used to mix lesser colours.
Quinacridone gold appears to have been special even by the standards of the quinacridone family of pigments, which Bruce MacEvoy at handprint described as follows: “Among the miracles of modern industrial chemistry […] one must include the discovery and development of the quinacridones.”
Quinacridone gold PO49 is described at The Color of Art Pigment Database as “a mixed crystal phase of Quinacridone & Quinacridonequinone; C.I.Pigment Violet 19 (C.I. 73900) and C.I.Pigment Red 206 (C.I. 73920) co-precipitated. The exact derivative has not been disclosed.” Bruce MacEvoy at handprint claims “PO49 is another mixed crystal form of PV19 alpha and beta.”
Not surprising that the formula was proprietary, but still, what is it? And what is PV19?
Let’s look at our glossary of terms before discussing further.
Polymorphism describes the existence of a solid compound (containing more than one type of atom) in different crystalline forms differing in the way molecules are arranged in the solid. Polymorphs have different properties, which is why controlling their formation is crucial in pigment manufacture. Polymorphs are often referred to as crystal phases.
Crystal phase refers to a form of a solid in which the comprising atoms or molecules are arranged in an orderly fashion throughout. The crystal is said to have long range order, i.e. it is possible to predict with a high certainty what atom will be found at a specified position.
Powder X-Ray diffraction pattern is a graphical representation of the crystal phase features still distinguishable in the powder, i.e. after the crystal has been ground up, or if only a collection of minute crystals is available. X-rays are diffracted through a crystal like visible light is diffracted through a prism. The X-ray diffraction experiment records where the light ends up and how intense it is.
Solid solution describes a homogeneous mixture with structure and properties different to those obtained by simply mixing the components in the same ratio. It shows an X-ray diffraction pattern different to the sum of the patterns of its components.
First (amusing) bit of information is the mention how better red pigments were needed in the 1950s due to it having recently become a popular colour for cars.
The alpha, beta and gamma crystal phases of quinacridone and the method for controlling their formation represents the scope of the three DuPont patents, together with the characteristic X-ray diffraction patterns each phase exhibits.
The crude quinacridone formed by oxidation of dihydroquinacridone (more on this later) is subjected to a milling process under defined conditions which generates a material fine enough to use as pigment, and with control over the crystal phase. The process looks deceptively simple: quinacridone is mixed with salt, mill balls or cylpebs (little cylinders) and nails (the latter to prevent caking), and the mixture rotated until the pigment particles are fine enough. The material is then washed with dilute sulfuric acid to remove the salt and any metal contamination introduced by the milling process. The quinacridone can then be used as a paste, or washed with methanol (to remove water) and xylene (to remove methanol) before drying to obtain the powder.
The control over phase formation is achieved using solvents. Milling without DMF (dimethylformamide) or xylene converts quinacridone to the alpha form. Adding 25% xylene with respect to quinacridone during milling results in the beta phase forming. Adding DMF during milling or stirring quinacridone in DMF either prevents the gamma phase from turning to alpha, or turns other phases to gamma. Long story short, the gamma phase is the most stable and will form (predominantly) when sparingly soluble quinacridone crystallises from solution.
Such crystallisation also occurs in the high temperature oxidation of dihydroquinacridone with nitrobenzene-m-sodium sulfonate under alkaline conditions in a water/alcohol mixture. DuPont patented their way of making dihydroquinacridone.
The condensation of diethyl succinate was performed in Dowtherm A (23.5% biphenyl and 76.5% diphenylether) and after a quick water wash, the resulting diethyl succinyl succinate (name which I find terribly confusing) solution was treated with aniline and catalytic anyline hydrochloride and heated under vacuum (no further detail here). Another quick wash to get rid of the catalyst, a distillation to remove aniline and the resulting substituted dihydroterephthalate can be used as a suspension in Dowtherm. The discovery here is the closing of the quinacridone ring system by heating the dihydroterephthlate at a high temperature (which is why Dowtherm is used) in the absence of oxygen to give dihydroquinacridone. The last step is an oxidation to remove the extra two hydrogen atoms, which is preferably achieved using the soluble oxidant meta-nitrobenzenesodium sulfonate. The great thing about the (dihydro)quinacridones is they are so insoluble, and isolation by filtration provides pure products.
The background and chemistry covered, let’s return to the question of PO49.
Turning red into gold
A number of other developments made by DuPont over the following decade resulted in what is most likely PO49. This patent from 1964 describes a number of inventions which allowed for extending the range of colours quinacridone pigments can attain. First of them is the incorporation of a derivative of quinacridone called quinacridonequinone, which can be formed by extending the reaction time in the oxidation of dihydroquinacridone described above. This sounds like a cost effective solution – tweaking a known process rather than having to develop a whole new one.
A mixture of quinacridone and quinacridonequinone can be processed to form an intimate mixture either by milling or by co-precipitation from concentrated sulfuric acid in water (known as drowning). This mixture, on solvent treatment (DMF) converts to a solid solution. Solid solutions, having properties different to that of a simple mixture, can have a colour different to that of their components. A solid solution containing quinacridone and quinacridonequinone is reported to have a maroon colour, quite different to the reds that quinacridone alone can generate. By varying amounts of components and their nature (various other quinacridones) a whole range of colours shifting towards yellow or blue was reported. But no gold.
Finally, a technical improvement on the precipitation step from sulfuric acid afforded “a bright gold pigment.” This invention adds the sulfuric acid solution to a high velocity flow of water to form very fine particles, which is believed to favour solid solution formation. The resulting slurry is then heated to convert the solid mixture into the solid solution. There is no need for any solvent treatments, which makes the process even more attractive.
I would say the story of PO49 going out of production sounds plausible. For whatever reason, the automotive industry didn’t want PO49 anymore. No demand – I would imagine the amount of pigment used by industry is far greater than what artists go through – means no supply, even if to the outsider chemist the gold pigment does not stand out as anymore of a challenge than red. Quinacridones are still being produced, and since PO49 does not appear to have any special challenges associated, its demise really might be due to gold falling out of favour while red is still going strong. Perhaps PO49 was superseded; chemists are always mixing something up.