The yellowish goo that oozed from the tube is technically called "separation" and it probably constitutes a mixture of material composing the binder (the "medium"): typically gum arabic or gum tragacanth, possibly a stabilizer like alba gel (a refined clay), a preservative (like formalin or some other materials like the fungicide orthophenol-phenate), and of course water. Technically speaking, a paint is balanced when it is first manufactured, that is to say, the pigment (strictly speaking the insoluble colorant) is mechanically dispersed and spread uniformly throughout the entire volume of the binder. Various things like the specific gravity and wetting characteristics of the particular pigment, the fineness vs. coarseness of the mechanical dispersion process, the level and kind of suspension agents present used to stabilize the dispersion, and how the finished paint was stored in the tubes (e.g. temperature cycles during storage), all can affect whether a paint shows separation when you open the tube and squeeze out the contents. This statement, based on what I saw working in quality control as an artist/technical consultant with the old M. Grumbacher Company in New York for ten years, applies to oils, acrylics, and of course watercolors (both gouache and aquarelle). If you choose to use the thicker, imbalanced material within the tube as a paint, you might get unexpected results, such is flaking off the paint film, especially with gouache, since the discarded "goo" is actually mostly the binder. One solution is to open the tube at the back at the crimp, dig out the thickened "paint-like-mass" with a spatula, and mix all the "goo" back together into the mass. Then spoon it back into the tube and re-crimp the end. (If it is aluminum, you might need a pair of pliers to pinch the crimp tightly when you finished). The material in the tube is closer to being a balanced formula now, in fact, it could properly be called a paint again.
I hope this helps,
On Mar 24, 2010, at 2:38 PM, Consie Powell wrote:
> What do other folks do in this situation? I have just gotten some new tubes of paint (the particular ones are gouache, of very good quality, but this has happend with transparent watercolor too...) and a few of them are exhibiting a situation I've had happen before. When I go to squeeze paint onto the palette for the first time, out first comes a runny squirt of often yellowish goo, which I presume is gum arabic. Often it is mixed somewhat with the pigment, but is way more dilute than the pigment is supposed to be. Often I can tell that this is the case when I first open the tube, as I can see it. What do others of you do when this happens? (I'm presuming I'm not the only one who has seen this...)
> Consie Powell
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