Watercolour and Gouache: Gum, Honey, Glycerin and Flow

A binder that never stops being soluble

The defining property of watercolour is that its binder remains water-soluble

after it has dried. A dried pan can be rewetted and used; a dried acrylic

cannot.

This is a formulation choice rather than an inevitability, and it has

consequences in both directions. It is what makes pans possible, what allows a

passage to be lifted or softened after the fact, and what allows a subsequent

wash to disturb the one beneath it.

Gouache differs from watercolour principally in solids loading rather than in

binder chemistry. Higher loading gives opacity and the characteristic flat

matte surface; the binder is doing the same job in both.

The components that shape a wash

Two of these deserve emphasis because they are so often attributed elsewhere.

A pan that rewets readily and one that resists is usually a difference of

humectant, not of pigment or of age. And a wash that spreads evenly rather

than gathering into beads is behaving as a wetting agent allows it to; the

same pigment in a formulation without one behaves quite differently on the

same paper.

Paper is the other half

No account of a wash is complete without the surface receiving it.

A watercolour paper’s sizing governs how far and how fast water penetrates,

and therefore whether a wash sits on the surface and dries evenly or is drawn

in and dries with hard edges. Sizing is itself variable and is not disclosed

in comparable terms.

So the behaviour a painter observes is a joint property of a formulation and a

sheet, and attributing it to either alone will mislead.

The limits are the usual ones, and they bite hard here. Component identity and concentration are proprietary and undisclosed for artists’ products. Reformulation occurs without announcement, so a product-level statement decays with time.

There is also no published comparison of humectant and plasticiser systems

across artists’ watercolour ranges, and the long-term behaviour of a film whose

binder remains soluble — including how humectant content affects it over

decades — is not characterised.

What can be said is the structural point. In watercolour, more than in any

other medium, the handling is the formulation. A colour that behaves

differently from another of the same name is not a different pigment. It is a

different set of the components above.

Humectants and Plasticisers and Surfactants, Defoamers and Wetting Agents

cover two of those classes in their own right.