# Aqueous Media

Painting I · Colour, Perception and Materials · https://tryals.app/learn/painting-i/aqueous-media

## The Binder Is the Medium

Pigments travel between media almost unchanged. What changes is the **binder**, the substance holding pigment particles together and anchoring them to a surface.

| Medium | Binder | Dries by | Characteristic |
|---|---|---|---|
| Watercolour | Gum arabic | Evaporation | Transparent |
| Gouache | Gum arabic plus chalk | Evaporation | Opaque and matt |
| Egg tempera | Egg yolk emulsion | Evaporation, then curing | Fast-drying, hard |
| Acrylic | Polymer dispersion | Evaporation and coalescence | Flexible, water-resistant |
| Buon fresco | Wet lime plaster | Carbonation | Highly permanent |

### Refractive Index

**Here** is a core studio principle. A particle's opacity depends on how its **refractive index** compares to its surroundings.

* Air is **1.00**.
* Linseed oil is about **1.48**.
* Chalk is around **1.57**.
* Rutile titanium dioxide reaches **2.70**.

In air or water, chalk scatters light strongly and appears white. In oil, chalk is index-matched and turns nearly transparent. By contrast, titanium white differs from oil by **1.2** and covers aggressively.

### Specific Media Rules

**Buon fresco** is painted directly into wet lime plaster. The plaster cures by carbonation, locking pigment into the wall. Work must finish before drying occurs. Plaster is therefore laid in a **giornata**, a single day's patch.

**Acrylic** dries by coalescence. Polymer particles fuse into a solid film as water evaporates. This process requires a minimum temperature, usually around 9 degrees **Celsius**.

> **Common pitfall:** Over-thinning acrylic with water. Unlike watercolour, acrylic polymer is a particle suspension. Adding too much water prevents the particles from forming a continuous film. The dried paint will rub off as loose powder. Always use an acrylic medium for heavy dilution.

## Practice questions

10 of this lesson's 15 practice questions, with answers. The full set is in the app.

### 1. Rutile titanium dioxide has a refractive index of about 2.70 and chalk about 1.57. Linseed oil is 1.48. By what factor is the rutile's difference from the oil greater than the chalk's?

**Answer:** 13.6 (within ±0.5)

**Why:** By a factor of about **13.6**. The differences are 1.22 and 0.09, and since scattering rises with that difference, this single ratio explains why titanium white obliterates whatever is under it while chalk disappears into an oil film and does nothing but occupy volume.

Page: https://tryals.app/practice/painting-i/aqueous-media/rutile-titanium-dioxide-has-a-refractive-index-of-about-2-70-and

### 2. Which of these are true of watercolour and gouache?

A. Both use gum arabic as the binder
B. In watercolour the white of the picture is the paper rather than a pigment
C. Gouache dries darker than it appears when wet
D. Gouache carries added chalk and a heavier pigment load, which makes it opaque

**Answer:** A. Both use gum arabic as the binder; B. In watercolour the white of the picture is the paper rather than a pigment; D. Gouache carries added chalk and a heavier pigment load, which makes it opaque

**Why:** Gouache dries **lighter**, not darker, which is why matching a gouache passage to a dry one is so frustrating: the swatch you are judging is still moving. The rest of the list is the whole difference between the two media, and it is a difference of loading rather than of binder.

Page: https://tryals.app/practice/painting-i/aqueous-media/which-of-these-are-true-of-watercolour-and-gouache

### 3. Chalk reads as an opaque white in a water-based medium and almost disappears in oil, because linseed oil's refractive index is close to chalk's and air's is not.

**Answer:** True

**Why:** **True**, and it is the most useful single fact in the lesson. Chalk against air differs by more than half; against linseed oil by under a tenth. The particle is identical in both cases, so opacity is never a property of a pigment on its own but of a pigment and the medium around it.

Page: https://tryals.app/practice/painting-i/aqueous-media/chalk-reads-as-an-opaque-white-in-a-water-based-medium-and-almost

### 4. Complete the account of the aqueous media.

**Answer:** The binder of both watercolour and gouache is gum **arabic**. The area of wet plaster a fresco painter can finish in one working day is a **giornata**. Acrylic dries by water leaving and the polymer particles **coalescing** into a continuous film. In buon fresco, calcium hydroxide takes up carbon dioxide and turns into calcium **carbonate**, locking the pigment into the wall.

**Why:** The third blank is the one with consequences. Coalescence is physical and needs warmth, so an acrylic film made in a cold studio may never form properly however long it is left, unlike an oil film, which is waiting on a chemical reaction rather than on the weather.

Page: https://tryals.app/practice/painting-i/aqueous-media/complete-the-account-of-the-aqueous-media

### 5. A student paints a large acrylic canvas in an unheated studio in January at about 5 degrees Celsius. In March the film is chalky and lifts under a fingernail. How does the way acrylic dries account for the failure?

A. Water evaporated too slowly at 5°C, letting water-soluble binder redissolve rather than set.
B. Being below the minimum film-formation temperature stopped polymer particles from coalescing.
C. Low temperature prevented polymer dispersion curing via its essential chemical cross-linking.
D. Paint dried strictly from the surface down, trapping moisture beneath a skin that disintegrated.

**Answer:** B. Being below the minimum film-formation temperature stopped polymer particles from coalescing.

**Why:** Coalescence has a temperature floor, commonly quoted near 9 degrees for artists' acrylics, and below it the particles never fuse however long the water takes to go. The film is then loose pigment held by almost nothing. No amount of extra drying time repairs it, because the process that failed is physical rather than chemical.

Page: https://tryals.app/practice/painting-i/aqueous-media/a-student-paints-a-large-acrylic-canvas-in-an-unheated-studio-in

### 6. Arrange the stages of laying one giornata of buon fresco in order.

**Answer:**

1. Lay the rough arriccio coat and let it set
2. Transfer the cartoon for the area to be painted today
3. Lay the fine intonaco over exactly that area and no more
4. Paint into the wet plaster with pigments ground in water
5. Stop at the edge of the day's area and cut a clean join

**Why:** The last step is the discipline the technique imposes. The plaster decides when the day ends, so the join must be cut where it can be hidden — along a contour, a fold, an architectural edge — and a painter who works until the plaster catches has put the seam wherever the accident fell, for ever.

Page: https://tryals.app/practice/painting-i/aqueous-media/arrange-the-stages-of-laying-one-giornata-of-buon-fresco-in-order

### 7. Pigments remain chemically identical across media, yet watercolour is celebrated for transparency whilst gouache yields a flat, opaque film. What structural difference in the binder creates this divide, and what does it demonstrate about paint formulation?

A. Gouache relies on chalk to scatter light rather than oil
B. Chalk modifies optical behaviour without a pigment swap
C. Watercolour excludes filler to maximise pigment curing
D. Gouache incorporates extra gum to accelerate coalescence

**Answer:** B. Chalk modifies optical behaviour without a pigment swap

**Why:** Chalk functions as an extender whose refractive mismatch with water and air scatters light, creating opacity without altering the pigment itself. Gouache dries purely by evaporation, not coalescence, and watercolour requires no curing stage.

Page: https://tryals.app/practice/painting-i/aqueous-media/pigments-remain-chemically-identical-across-media-yet-watercolour-is

### 8. A wall of 18 square metres is to be painted in buon fresco by a painter who has been averaging 1.2 square metres a day. How many days of plastering and painting does that imply?

**Answer:** 15 (within ±2)

**Why:** **15 days**, and therefore fifteen joins to place. The arithmetic is the reason a fresco is designed around its seams from the start: the cartoon has to be divided into areas that can each be finished before the plaster sets, and the divisions have to fall where a contour will hide them.

Page: https://tryals.app/practice/painting-i/aqueous-media/a-wall-of-18-square-metres-is-to-be-painted-in-buon-fresco-by-a

### 9. An egg tempera medium is made by mixing one yolk of about 15 millilitres with an equal volume of water, then combining that medium with pigment paste in equal volume. How many millilitres of ready paint does one yolk produce?

**Answer:** 60 (within ±2)

**Why:** **About 60 ml.** Fifteen of yolk plus fifteen of water gives 30 of medium, and 30 of medium plus 30 of pigment paste gives 60. The yolk volume is approximate and varies with the egg, which is why tempera recipes are given as ratios rather than as quantities.

Page: https://tryals.app/practice/painting-i/aqueous-media/an-egg-tempera-medium-is-made-by-mixing-one-yolk-of-about-15

### 10. Sort each medium by its binder.

**Answer:**

- Bound by gum arabic: Watercolour, Gouache
- Bound by an egg or polymer emulsion: Egg tempera, Acrylic
- Bound by the lime plaster itself: Buon fresco, Lime wash on a freshly plastered wall

**Why:** The third column is the odd one and the reason fresco is taught separately. There is no binder in the paint: the wall becomes the binder as it carbonates, so the painting cannot be moved, cannot be corrected once the plaster has caught, and cannot use a pigment that alkali would destroy.

Page: https://tryals.app/practice/painting-i/aqueous-media/sort-each-medium-by-its-binder