The case for acetate

Pick up a good pair of acetate glasses and the first thing you notice is not the shape. It is the weight, and then the warmth. The material sits in the hand like something made rather than merely produced, and that is no accident. It comes down to what acetate is, and how it is worked.

Cotton, wood and colour

Cellulose acetate begins with plants. Its raw material is cellulose, taken from cotton linters, the short fibres left clinging to the seed once the long ones have gone for spinning, and from wood pulp. Treated, then blended with plasticisers and pigments, it becomes a dense, faintly translucent material that can be made in almost any colour or pattern, from clear crystal to the deep mottled browns of havana.

Because the colour is mixed into the material rather than painted on to it, it runs all the way through. A tortoiseshell pattern is not a print but shades pressed together, so no two frames are quite alike, and the pattern seems to move with the light.

Cut, not moulded

Many plastic frames are injection-moulded: molten material is forced into a mould and comes out shaped in seconds. Acetate is worked the other way round. It arrives as blocks or sheets, and the fronts and temples are cut from it, then shaped, bevelled and fitted. Many frames also have a thin metal core running through each temple, sometimes visible in translucent colours, for strength.

Then comes the polishing. Cut acetate is tumbled, often for days, in drums of small wooden pieces and polishing compound, and finished by hand against a buffing wheel. That slow work is what gives acetate its depth: a shine that seems to sit beneath the surface rather than on it, and edges soft enough to run a thumb along.

Two more things follow from the material. Acetate feels warm, because unlike metal it does not draw heat quickly from your skin, so there is none of that cold first touch. And it softens with gentle heat, which means an optician can warm the temples or the bridge, ease them into a better fit, and let them set. A frame that can be adjusted is a frame that can be made your own.

Metal is cold until you warm it. Acetate meets you halfway.

Living with it

Acetate asks very little, but it does have preferences. Rinse it in lukewarm water with a drop of mild soap and dry it with a soft cloth. Keep it out of hot cars and off sunny dashboards, because the warmth that lets an optician adjust it will, given long enough, bend it out of shape on its own. Keep nail varnish remover well away, since acetone marks the surface almost at once, and be wary of perfume, hairspray and insect repellent.

Take your glasses off with both hands to keep the hinges true, keep them in their case rather than a pocket, and set them down on their folded temples, never on their lenses. If the shine dulls over the years, acetate can usually be polished again, which is one of the quiet reasons a good frame lasts.

What to look for

When you are judging a frame, start with the hinges. Open and close them a few times: the movement should be smooth, with a little resistance, and the temples should fold flat and rest evenly side by side. Hinges with more interlocking barrels, five or seven rather than the usual three, are generally the sturdier kind. On many classic frames the hinge is anchored by rivets that pass right through the front, and pins at the corners, such as the twin silver pins on Lumix or the diamond studs on Vertex, carry on that tradition.

Then look at the finish. Run a finger around the inside of the rims and along the bridge: there should be no ridges, no tool marks and no rough edge where the lens sits. The polish should be as even on the inside of the frame as on the outside, because that is where hurried work shows first. Hold the frame up to the light and check that the two sides are symmetrical, and that any pattern runs cleanly from one to the other.

None of this is complicated. It is simply what separates a frame you wear for a summer from one you keep for years. Lumix and Vertex in Smoke / Amber, our first edition, have sold out; the rest of the family is coming in Collection II, and you are welcome to join the list. For the lens inside the frame, see On choosing a tint.