STOCKMAR Lightfastness

STOCKMAR Lightfastness and Wet-on-Wet Painting

Wet-on-wet watercolour painting is one of the quiet rituals of a Steiner classroom or life at home - a jar of water, a well-loved brush, colour spreading softly across damp paper. The paintings that come out of these sessions often end up pinned to a wall, tucked into a memory box, or kept as part of a family's story or yearly rhythm.

Which is why the pigments behind the colour matter more than people tend to think.

This guide was put together by our Mercurius Australia team, prompted by questions that have come up within the Australian Steiner teacher community.

The Blue Wool Scale

This is the international standard used to test how a colour holds up under light exposure - it runs from 1 to 8, with 8 representing a colour that shows no visible change at all after exposure. STOCKMAR pigments sit at 7-8 on this scale, the top tier used across the wider paint and pigment industry.

The scale isn't linear either - each step up roughly doubles the resistance to fading, so the gap between a mid-range rating and a 7-8 rating is larger than the numbers alone suggest.

The other scale you'll see - ASTM

Artist paint manufacturers also reference a second scale - ASTM ratings, running from I to V. A rating of I means excellent lightfastness, and it's the classification that corresponds to that same 7-8 Blue Wool range. It's the standard used across fine art materials globally to indicate a pigment expected to hold its colour for decades under normal display conditions - out of direct sun, as most framed or displayed artwork is kept.

What lower ratings actually mean

Further down the scale, ratings start to describe something quite different. A II-rated pigment is considered very good, but shows gradual change over a long period. A III-rated pigment is where fading becomes more noticeable - and particularly so when the pigment is diluted rather than used at full strength.

That distinction between full strength and diluted colour turns out to matter a lot for the kind of painting most families and Waldorf classrooms actually do.

Why this matters for wet-on-wet painting

Wet-on-wet technique dilutes pigment heavily by design - the wash, not the masstone, is doing most of the work on the page. Children are taught to mix colour into water rather than apply it thickly, letting it flow and blend on wet paper.

This means a pigment's performance in dilution is arguably more relevant here than in almost any other painting context - because that's exactly the strength most of these paintings are made at. A paint that tests well at full strength but fades when watered down isn't well suited to a technique built around dilution.

Where STOCKMAR sits

Every STOCKMAR pigment across the range - watercolour, opaque paint, wax crayon - is developed and selected to meet that 7-8 Blue Wool, ASTM I standard, including at the diluted strengths used in wet-on-wet work. Lightfastness is only one of several criteria STOCKMAR pigments are chosen against - alongside safety for children, insolubility in water, and colour brilliance - but it's the one that determines whether the painting your child brings home today still looks the same in five years, or fifteen.

It's been part of the STOCKMAR standard since 1922 - colour that stays legible, stays true, and holds up to the kind of everyday light a family home or classroom naturally gets.

Why the rating alone doesn't tell the whole story

Here's something worth understanding if you start comparing lightfastness ratings between brands - not all ratings are earned the same way.

Many lightfastness ratings are based on testing a pigment at full, undiluted strength - the condition it performs best under. That's a reasonable starting point, but it doesn't always predict how the same pigment behaves once watered down into the pale washes typical of wet-on-wet painting. A pigment can test as reliable at full strength and still fade more than expected once diluted, and the rating on the label doesn't always distinguish between the two.

This matters at every level of the scale, not just the lower ratings. Even excellent pigments have a dilution point where fading eventually becomes visible - top-rated pigments simply have a wide margin before that point is reached, while lower-rated pigments start much closer to it, or are already past it in typical use.

It also means a mid-range rating like ASTM II can mean quite different things depending on the pigment behind it. Phthalo blue and phthalo green, for instance, carry a II rating but are among the most dependable watercolour pigments, holding their colour well even diluted. Others earn a II specifically because they soften once watered down, despite performing well at moderate strength. The number alone doesn't tell you which kind of II you're looking at - what matters is how the pigment performs at the strength it's actually used at, which for wet-on-wet work is thin and diluted, not full strength.

Photo Courtesy: Linuwel Steiner School

FAQs

What does "lightfastness" mean for watercolour paint?
Lightfastness is how well a pigment resists fading when exposed to light over time - the higher the rating, the longer a colour stays true rather than washing out or shifting tone

What lightfastness rating do STOCKMAR paints have?
STOCKMAR pigments are rated 7-8 on the Blue Wool Scale, which runs from 1 to 8 - 8 being a colour considered not to have altered from the original at all

What's the difference between the Blue Wool Scale and the ASTM rating?
Blue Wool is the original international testing scale, running 1 to 8. ASTM is the scale most commonly used on artist paint labels, running I to V. A Blue Wool rating of 7-8 corresponds to ASTM I, the top classification for lightfastness

Why does lightfastness matter more for wet-on-wet painting?
Wet-on-wet technique relies on diluted washes rather than full-strength colour, and diluted pigment is where fading shows up first and most noticeably. A pigment that performs well at full strength doesn't always perform as well once watered down, so lightfastness in dilution matters more here than in most other painting techniques

Will my child's paintings fade over time?
Kept away from direct sunlight, as with any artwork on paper, STOCKMAR paintings are formulated to hold their colour for decades. The 7-8 Blue Wool rating applies to the pigments in the paint whether at full strength or diluted, so a wash painted thin should still hold up in the same way as one painted strong

Does a high lightfastness rating mean the paint is toxic-free or safe for children?
No, lightfastness is a separate property to safety - it only measures resistance to fading under light. STOCKMAR's paints are separately formulated and tested to meet safety standards for children, non-toxic and free from common allergens

Are all lightfastness ratings tested the same way?
Not always. Many lightfastness ratings are based on a pigment at full, undiluted strength - which is the condition pigments perform best under. Once diluted, some pigments fade faster than their rating alone suggests, which is why the diluted-wash performance of a paint matters more than the label rating on its own, particularly for wet-on-wet technique

Does an ASTM II rating mean a pigment isn't reliable?
Not necessarily. Some pigments rated II are highly stable in practice - phthalo blue and phthalo green, for example, are considered some of the most dependable pigments in watercolour and often perform close to top-tier lightfastness even when diluted. Others carry a II rating specifically because they're known to fade a little once heavily watered down, even though they hold their colour well at moderate strength. The rating alone doesn't tell you which kind of II you're looking at - it's worth knowing the individual pigment behind the colour