Cobalt blue gets talked about as though it were a single pigment. Walk into any decent art shop and you will find at least three blues sitting under that name, and they behave nothing alike. One is a modern workhorse, one is a centuries-old gamble, and one is largely a historical curiosity that still turns up in old master palettes and heritage paint ranges.

If you paint in watercolour, oils or acrylics, knowing which is which saves money and prevents disappointment. A blue that looks perfect on day one can fade within a season if you have chosen the wrong one, and a blue with low tinting strength can vanish the moment you mix it with white. What follows is a practical comparison, based on how these pigments actually behave on the palette and on the paper or canvas.

Cerulean blue: the modern standard

Cerulean is the cobalt blue most painters mean when they use the word. Chemically it is cobalt(II) stannate, made by combining cobalt and tin oxides at high temperature, and it was first produced commercially in the nineteenth century. It sits in a genuinely useful place on the colour wheel: a clean, slightly green-leaning blue that mixes into soft skies without turning muddy.

Its great virtue is opacity. Unlike ultramarine, which is transparent and loves to granulate, cerulean covers. In watercolour it sits on the surface of the paper and can be lifted with a damp brush if you work quickly. In oils and acrylics it has real body, which makes it excellent for building pale, luminous skies and for the soft edges of distance.

Lightfastness is where cerulean earns its reputation. Genuine cerulean made with the modern pigment is among the most permanent blues available, holding its hue and value over decades. The catch is price. Cobalt is an expensive raw material, and good cerulean reflects that. Tubes are noticeably dearer than most other blues of similar volume.

Where cerulean wins

• Skies, particularly the soft, high-value blues of a British summer
• Mixing with yellows for clean greens that do not sulk
• Watercolour work where opacity and lifting matter
• Any painting intended to last a long time without shifting

Smalt: beautiful, cheap and quietly unstable

Smalt is the old one. Ground cobalt glass, usually tinted with cobalt oxide and then milled to a powder, it was the standard blue of European easel painting from roughly the sixteenth to the eighteenth century, long before cerulean existed. You can see it in the skies of many old master paintings, and in plenty of them you can also see what went wrong.

The problem is particle size. Coarse smalt gives a strong, deep blue; grind it finely, as painters often did to make it easier to work with, and the colour weakens dramatically. Worse, it is not reliably lightfast. In oil, particularly where it is bound in a drying oil and exposed to light, smalt can lose saturation and shift towards grey. Some paintings show skies that have gone from blue to a ghostly grey-green over centuries.

Tinting strength is moderate at best and depends heavily on the grind. Handling is awkward: it is a glass, so it is abrasive, and it behaves quite differently from a modern synthetic pigment. In watercolour it is rarely worth the trouble. In oils, some painters use it for historically informed work where the aim is to reproduce a period palette.

Where smalt makes sense

• Reproducing a historical palette for study or conservation-style work
• Laying in muted, grey-blue underpainting where permanence is not critical
• Understanding how old paintings have changed over time

If you are painting anything you want to keep, treat smalt as a pigment to research rather than to rely on.

Zaffre: the raw material, not the paint

Zaffre is often listed alongside cerulean and smalt as though it were a competing blue, but it is really the ingredient rather than the product. The name refers to the impure cobalt oxide material produced by roasting cobalt ore, historically the starting point for making smalt and for the blue decoration on ceramics.

You will not normally find zaffre sold as an artists' pigment in a modern tube. It appears in historical recipes, in ceramic glazes and in discussions of how cobalt blues were made before industrial chemistry. Where it does show up in a paint range, it usually signals a colour made to evoke that older, slightly dull, granulating blue rather than a true raw material.

The practical point for painters is simple: if you see zaffre named on a tube, read the pigment index number on the label. That tells you what you are actually buying. If it is a cobalt-based pigment you will see a code beginning with PB (pigment blue), and the number will let you compare it directly with cerulean or with other blues you already know.

Comparing them side by side

The differences matter most in three areas: permanence, strength and cost.

1. Lightfastness. Modern cerulean is excellent and can be relied on for permanent work. Smalt is unreliable and can fade or shift. Zaffre, as a raw material, is not a paint and has no meaningful lightfastness rating for artists' use.
2. Tinting strength. Cerulean is moderate to strong and holds its character when mixed with white, giving clean pale blues. Smalt is weaker, especially when finely ground. Zaffre offers no consistent comparison.
3. Cost. Cerulean carries a premium because of the cobalt content, but it delivers value over time. Genuine smalt is scarce and often expensive for what it is. Historical names attached to modern paints can carry a price that has little to do with pigment quality.
4. Handling. Cerulean is opaque, easy to control and forgiving. Smalt is abrasive, variable and awkward to grind or mix. Zaffre, being an ingredient, has no handling characteristics of its own.

Tips for choosing and using cobalt blues

Learn to read labels. Every reputable manufacturer prints a pigment index number, usually a code such as PB35 or PB36, alongside the colour name. Names are marketing; codes are chemistry. Two tubes with the same name from different brands can contain completely different pigments, and the code is the only reliable way to tell.

Buy the smallest tube first. Cerulean is expensive enough that a large tube is a real commitment. Start small, paint a few skies and a few greens, and see whether the opacity suits how you work. Some painters find they reach for it constantly; others find ultramarine plus a touch of a green does the job for less money.

Test for yourself before trusting a new pigment in a finished piece. Paint a swatch at full strength, a mid-tint with white, and a thin wash, then leave it on a windowsill for several weeks. It is not a laboratory test, but it will show you how a pigment behaves in your own medium and whether it shifts.

Mix deliberately. Cerulean with a warm yellow gives fresh, slightly chalky greens that work well for foliage in distance. With a little magenta or alizarin it moves towards useful violets and greys. Because it is opaque, add it to a mix in small amounts and build up, rather than dumping it in and trying to correct afterwards.

Keep historical pigments in their place. Smalt and zaffre are fascinating, and understanding them makes you a better reader of old paintings. That does not make them good choices for a commission or a piece you intend to sell. Use modern, well-labelled pigments for work that needs to last, and enjoy the old ones as a study.

A practical shortlist

For most painters, a single tube of modern cerulean covers nearly everything a cobalt blue is asked to do: skies, distance, clean greens and soft greys, with permanence you do not have to worry about. Keep ultramarine alongside it for transparent, granulating passages and for deep mixes, and the pair will handle almost any blue you need.

Smalt and zaffre are worth knowing about and worth reading about. Buy them if you are curious, if you are working on a historical project, or if you simply want to understand what earlier painters were dealing with. Just do not expect them to behave like the cerulean in your paintbox. They are different materials from a different era, and treating them as interchangeable is the fastest route to a painting that does not look the way you intended.

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