What a conservator does that no software can

Two quite separate trades share one word. A photograph conservator treats the physical thing — paper, emulsion, mount, and the damage in them. A digital restorer works on a copy and leaves the object precisely as it was found.

Neither can take on the other's work. No conservator can restore a corner that went missing in 1962, and we cannot flatten anything, part anything, or do a thing about an acidic mount.

Parting prints that have dried to one another, or a print that has bonded to its frame glass — done with humidity, time and occasionally solvents, and unforgiving of error.

Relaxing a rolled or badly cockled photograph through controlled humidification and slow drying under weight.

Work on the paper itself: filling losses with matched fibre, mending tears from the reverse with tissue and starch paste, consolidating a print that is shedding.

Lifting a photograph off an acidic mount or an album page it was glued to, which is common, delicate and frequently the reason people ring a studio in the first place.

Surface cleaning under magnification, which bears no relation to wiping.

Conservation is charged by the hour, and a single treatment commonly runs from around a hundred pounds to several hundred, with a wait of weeks. That is not a complaint about the price. It is one trained pair of hands at a bench, working on your object and nobody else's.

The sums only work per object. For the one photograph that outranks the others — a cased daguerreotype, the only surviving portrait of a great-grandparent, something with real market value — it is money well spent. For a whole tin, the arithmetic has never worked and never will.

The print sheds, powders or flakes when it moves at all.

It is stuck to glass, to another print or to an album page, and you want the object back rather than a file.

It is a cased photograph — a daguerreotype, ambrotype or tintype. These are chemistry rather than paper, they tarnish, and household cleaning ruins them on the spot.

It carries monetary or archival value, where the treatment and its written record form part of what the thing is worth.

The damage is already in the print, and what you want back is the photograph rather than the damage.

There are more photographs than there is budget for one of them. Two hundred prints mended well enough to enjoy beats a single print treated immaculately.

You want it this week, with the print never leaving the house.

You want to see the result before committing, which is the one thing a bench cannot offer.

The professional bodies maintain public registers of accredited conservators, searchable by region and speciality, and that is the place to start. County record offices, larger museums and university archives all use conservators and will generally tell you whom.

Family history societies are the other good source, since somebody in the branch has invariably had something treated and will say whether it was worth it.

The question to ask on the telephone is what is proposed and what of it is reversible. A good conservator answers that at length and is pleased to be asked.