How pigment becomes paint on a slab
A slab of ground glass, a glass muller, a knife and a sieve are the whole kit. This page walks one pigment from a dry heap to a labelled tube, stage by stage, and then answers the two questions every first batch raises: how much can be made at once, and why it came out too stiff.
Dry heap: Start with less than you think
Tip a modest heap of pigment onto the slab, no larger than a walnut for a first batch. Pigment on a slab spreads to twice its heaped size the moment it is wetted, and a heap that fills the slab cannot be mulled, only pushed about. The raw sienna on the wall above is the pigment we teach with, because it is transparent enough to show every stage and forgiving enough to survive a heavy hand.
Sieved: Break the lumps before the binder touches them
Earths arrive with a few clumps, and a clump wetted becomes a hard bead that no muller will open. Pass the heap through a sieve of a fine mesh onto the slab first. The sieve is the cheapest tool on the shelf and the one most often skipped; skip it and the first tube will carry grit for ever.
Wetted: Drops, not pours
Add the binder a few drops at a time and work it in with the knife until the pigment is a stiff, dull crumb that just holds together when pressed. It should look too dry. Every pigment has its own appetite, given on its entry in words such as thirsty or dense, and the crumb stage is where you learn it: a dense red ochre wets with a few drops, a thirsty terre verte wants many more.
Figure of eight: The muller moves, the pigment stays
Set the muller on the crumb and push it in slow figures of eight with the weight of your arm, not your shoulder. The flat of the muller shears each particle against the slab; the figure carries the paste outward and back so nothing sits in one place. Keep the muller flat. Tilting it grinds the edge into the glass and does nothing to the paint.
Gathered: Scrape it back to the middle
After a few passes the paste has crept to the rim in a ring. Gather it with the knife back to the middle, scrape the face of the muller clean into the heap, and go again. Gathering is half of mulling; a batch that is never gathered is only half ground however long the muller runs.
Second pass: Lighter, longer, listening
The second and later passes go with a lighter hand. The paste will start to feel slick rather than gritty and the sound of the muller changes from a whisper to nothing. Add a drop or two more binder only if the paste tears instead of flowing; most first batches need less than the hand wants to give.
Ground: Judging when to stop
Lift a little on the knife tip and draw it thin across a white tile. A ground paste draws out as a smooth, even film with no specks and a short, buttery break when the knife lifts. Grit shows as streaks; unwetted pigment shows as pale dust at the edges. If either appears, gather and go again. Mulling further than this gains nothing and, in the coarse minerals, loses colour.
Tubed: Filling and naming
Fill an empty tube from the open end with the knife, tapping it down on the bench every few knife-loads so no air is trapped. Fold the end over twice and crimp it flat. Then the label: pigment, colour-index name, binder, and the day. A tube without a label is a mystery in a month.
How much can be made at once
As much as the slab can carry in a single gathered heap, and no more. On a slab the size of a large book that is a heap you could hide under one hand, which mulls into enough paint to fill a small tube or two. Doubling the heap does not double the paint; it produces a ring of half-ground paste round a lump that never sees the muller. A painter who wants a large tube of one colour grinds it in several batches and scrapes them together at the end, which also gives the same paste throughout instead of one stiff end and one runny one.
The oil-bound colours can be mulled a batch at a time over a week and stored under a skin of oil in a jar until there is enough to tube. Watercolour is mulled straight into pans. Tempera is mulled in water only, kept as a paste, and mixed with yolk on the day.
Why a first batch is usually too stiff
Because the crumb stage looks wrong. A properly wetted pigment before mulling looks like damp sand, not like paint, and the instinct is to add binder until it looks like paint. Then the muller starts, the shearing releases the binder that was trapped between dry particles, and the paste that seemed stiff goes slack. So the beginner who resisted that instinct ends up with a stiff batch, and the beginner who gave in ends up with a runny one; the stiff batch is the better mistake, because a drop of binder fixes it and nothing fixes runny paint except more pigment and another twenty minutes.
The second reason is the pigment's own appetite. The entries on the earth page say in words how much each colour drinks. Raw umber and terre verte are thirsty; red ochre and Mars black are not. A recipe that suited one will misjudge the other, which is why we do not print a single proportion and why a swatch card in your own binder is worth more than any table.
Tools on the shelf
Mullers in a few hand sizes, ground-glass slabs, flexible painting knives, sieves in several mesh counts, empty tubes, jars and labels. Ask for what you need alongside the pigment and it is posted in the same box.
