PASTELS

John Hersey is an artist. His burning interest is in light. ''Artists are usually interested in things,'' he says. But the charcoal drawings he was making -- a dozen years ago now -- were, he realized, ''all about light. Nothing to do with landscapes, roads, and trees.'' And ''the whole point about light,'' he explains, ''is that it's all-inclusive. There's nothing outside it.''

His charcoal drawings were only in tones ranging from black to white. But he had arrived at the point where he ''wanted to know, supposing I was to use color instead of black and white, what colors would I use?''

So he decided to buy some pastels to explore ''what color is about -- why there is such a thing, what its meaning is, what is interesting about it if anything.''

Avoiding certain brands he already knew he disliked for their variable hardness, scratchiness, or crumbliness, he bought 130 pounds worth of pastels by the French company Sennellier. This established company is large enough, no doubt, to survive his rather unflattering conclusions about their pastels: ''I found that they were absolutely meaningless in relation to what I saw. They had no cohesion about them.''

Harmonizing colors

And that was when he began to make his own pastels. Today, artist Hersey is also a maker of pastels for other artists. In fact, his only problem now is finding the time to use his own pastels.

He calls them Unison Colour Soft Pastels, and it is not just a good advertising name for them. The idea of unison is crucial to the kinds of colors he makes. The colors he makes are translations of light into pigment (sometimes mixing up to eight pigments to make one subtle color). But more important, he sees the relationship of his colors in musical terms.

He makes standard sets of pastels each consisting of 18 colors. Each set is based on a color. (He will also make additional separate colors specified by a customer when asked.) The ''base'' color, such as ''red earth,'' ''yellow-gold,'' ''blue-green,'' and so on -- he likens to the tonic in harmonic music.

''Every note in the scale,'' he says, ''is only meaningful in relation to the tonic.'' If the tonic changes, so do all the notes relating to it. And Hersey's colors, like musical notes, are continually being changed.

He approaches his pastelmaking as an artist rather than as a chemist or a mass-manufacturer, and his colors are ''in a continuous state of flux.'' It is a disarmingly personal business. His eye tests the colors against the set standards before the pastels are packed and sold. But more importantly, it is his eye that is continually dissatisfied with the standards -- and keeps rethinking them. When Hersey decides to change the base color (which is usually, but not always, No. 9 in a set), all the other colors in the set change, too.

He stores examples of every color he has made over the eight years since Unison Colour began and can always match them if required. But the printed charts buyers are given for reference are ''out-of-date by the time they are printed.'' The inks used in these charts are approximate at best; serious buyers can purchase hand-drawn charts made with the exact colors.

Unison Colour pastels, which are hand-rolled in three sizes (giant, standard, and small) and in about 240 colors, are beginning to encircle the globe from the remote Northumberland rectory and coach house where they are made and marketed by a minute staff: Hersey, his wife, Kate, and, currently, two assistants with occasional help from the Hersey's young son. They can be bought directly from Unison by mail order. They are also sold through distribution centers and shops in Britain, Holland, France, Germany, Norway -- and are arriving in the US this summer courtesy of an agent in Wisconsin.

Small company, big product

Though Unison is a small operation among such giants as Rembrandt pastels (made by Talens), Rowney, and Sennellier, it is making its mark. According to artist Tom Coates, who is President of Britain's Pastel Society, Rowney is currently, ''making a reassessment of their pastels -- and they used Unison as a comparison.''

Pastel-makers other than Unison grade their colors, according to Hersey, by ''mechanical'' additions of white or black to a pure pigment, in order to make the color lighter or darker -- a ''stepped reduction of tints.'' Unison colors do have a tonal range, but the aim is to achieve colors with as little white or black added as possible, but made with mixtures of pure pigments.

Mr. Coates describes it this way: ''He's got the whole spectrum of color, and he's going out to the harmonies and hues by color-mixing each color.'' He compares Hersey's range of colors to the color-wheel idea: If you spin a color-wheel, all the colors appear to mix into a greyish-white. In other words -- white light is composed of all the pure colors of the spectrum.

Hersey says that his colors ''have a scale that does not go from A to B in a line [like other manufacturers' colors], but they go around. A cycle. A complete thing like a diamond.''

Apart from this different philosophy of color at the back of his pastelmaking, Hersey also aims to make the texture of his pastels as consistent as possible. Some colors of some makes are inclined to return to dust at the slightest touch -- they seem to be held together only by the paper around them. Unison pastels, though soft, are not as brittle as this.

Coates likes the ''feel'' of these hand-rolled pastels, and also the choice of size. Sennellier makes a limited range of ''giant'' size (4-inch) pastels, but Coates finds the colors offered ''very crude, very basic in color'' while Unison's 4-1/2 inch giants are available in all their subtle hues.

Tim Appleby, another artist who uses some Unison pastels, particularly likes these large pastels. But best of all is that they ''will make up a color for you specially.''

Sometimes, however, an artist asks for a color -- a bright geranium pink for example -- and runs up against Hersey's aesthetic fastidiousness.

''We tell them,'' he says, ''that such very bright and rather brash colors don't fit into our scheme of things because the pigments are made in factories out of organic chemistry. Some of these chemical pigments don't mix with water'' -- and water mixed with pigment is the very simple way in which Unison pastels are mostly made.

These are really excuses, however. The real reason is distaste. While Hersey is content that artists buy his pastels and use them in any way they choose, he still keeps intact his personal reasons for making them and not-so-secretly wishes other artists were as interested in light as he is. Light rather than geraniums. ''We're not interested in geraniums, you see,'' he says.

One day, presumably, he will make his own images with his own pastels. In the meantime, it is perfectly clear that making pastels, and continually refining their color relationships, has actually become his art form.

I suggested he might simply accept this and show his pastels in an art gallery as art.

''That would be very pretentious,'' he chipped in dismissively. But then he added more thoughtfully: ''But some people who buy our pastels have told me that sometimes when they are feeling depressed they simply open the box and look at them.''