Gouache techniques

​In dealing with some of your questions regarding fixatives and framing options, we would refer you to these older MITRA threads here (you may also find more info if you enter terms like “fixative” or “framing” into the search field): 
1) https://www.artcons.udel.edu/mitra/forums/question?QID=535
2)  https://www.artcons.udel.edu/mitra/forums/question?QID=343

As for your specific questions regarding technique I will contact some of the other moderators so that they can hopefully address some of these questions.

So I can try to answer a couple of the first questions and possibly question no. 5. First of all gouache was originally opaque watercolor, either opacified via high pigment load or by the addition of certain filles (like chalk for example). More recently gouache has been bound with things like dextrin, which is a modified starch (as well as other possible binders). This is sort of immaterial when it comes to its handling techniques, at least in my opinion. But others might be able to prove me wrong on that one 🙂

So for no. 1 it seems in both of these examples that the ink being used is not “co-mingling” with the gouached per say and therefore their different solubilities are not relevant. However, one could use a very standard “India ink” which is generally composed of lamp black bound in a shellac-soap binder. This is not soluble in water and can be purchased in almost all art stores.

For the rest of your questions it seems that Kristin has already guided you to other relevant threads. Please get back to us after reading these over should you have additional questions. 

But as for no. 5….Yes I believe gouache can be mixed with acrylic painting. I do not know of any negative long-term effects but perhaps some of our colleagues at Golden can weigh in on this one.
Brian Baade

In terms of technique, the key difference between gouache and watercolor is whether the source of white in the painting is paint or paper. Depending on the degree of orthodoxy one invests in the process, mixing with white is avoided in transparent watercolor.
A traditional, transparent watercolor painting is constructed with strategic use of the paper as the source of white which reflects through the paint, preserving the full luminosity of colors. In transparent watercolor, white paint is generally reserved for “heightening” passages, meaning the white paint is used to exceed the brightness of the paper (e.g. reflections on water, glass or metal). Overpainting is limited in transparent watercolor, though beginners sometimes employ a more incremental approach where passages are developed gradually from multiple layers. Experienced painters usually eschew this method in favor of bold, deliberate applications.
Gouache technique primarily involves mixing colors directly with white paint, though almost all gouache painters also use transparent applications, with thinner layers denoting background, and thicker passages projecting forward. 
When gouache is layered, darker colors tend to show through lighter ones. This can be frustrating to artists new to the medium, because it limits correction and overpainting, but experienced gouache painters learn to incorporate this property into the process. Gouache is also prone to drying darker than its apparent wet color, but again, this is something the artist masters with experience. Beginners sometimes manage this effect by applying light colors first, and work gradually toward darks. One “cheat” that’s strictly for beginners is mixing PVA glue with the paint to reduce intermixing (essentially making a poor-man’s vinyl emulsion paint).
One exciting property of gouache is the difference in body between straight from the tube and diluted with water. Gouache is sticky and ropy from the tube, but becomes creamy when water is added on the palette. By dragging undiluted paint over dry passages, broken impasto-like effects can be created. 20th century illustrators often used this technique to highlight figurative forms. 
One important factor to consider with gouache is the degree of optical color change induced by fixative. Even a light coating of any sealant or varnish will deepen colors dramatically, irreversibly boosting value contrast well beyond the original appearance. 

​Regarding India ink, yes, W&N Black Indian Ink is a waterproof, shellac-based formula, as is Higgins #4415. Avoid “fountain pen india ink”, which is not waterproof, and may be dye-based rather than pigmented, depending on brand. 
Sometimes, it is possible to whisk a light watercolor wash over gouache that is very dry, but it quickly wets and intermixes with a second application. It works best when the gouache layer has enough white pigment to quickly draw the water out of the wash without wetting the paint beneath.