The slides and negatives are the part nobody has dealt with.

A transparency is the original, and it is a thin film of dye.

A slide is a positive transparency in a mount. A negative is the inverse. Both are the original image — unlike a print, which is a copy — and that changes the stakes: if the transparency fades, the image is gone, because there is nothing behind it to go back to.

Three materials matter:

Colour transparency (slide) film

Kodachrome, Ektachrome, Fujichrome, Agfacolor. Dye images on a plastic base, usually 35mm, mounted in card, plastic, or glass. The most at-risk material in family collections: the dyes fade in light and continue to shift slowly in the dark. Kodachrome is the most stable of them; Ektachrome and the amateur Agfa films fade noticeably faster, and a red cast in an old Ektachrome slide is dye loss, not warmth.

Colour negative film

Kodacolor, Vericolor, and the drugstore films. Orange-masked, designed to be printed. Far more stable than slide film in the dark, which is why a 1970s negative often prints better now than the print made at the time. The prints were the fragile part, not the film.

Black-and-white negative film

Silver image on a plastic base. Effectively permanent in cool, dry, dark storage. Its problem is physical, not chemical: scratching, dust, and the base itself.

The base is where the chemistry gets uncomfortable. Nitrate film (roughly 1889 to 1951) decomposes into nitric acid, smells of vinegar and camphor, and is flammable enough that archives store it separately. Acetate film (the "safety" stock that replaced it) fails more slowly into acetic acid — the vinegar syndrome that makes old negatives smell sharp and curl. Polyester (1950s onward, marked ESTAR) is stable and is what you want to find. You can identify the base by the edge printing on the film, and by smell.

The mount and the edge printing date the film better than the picture.

Nobody photographs their photographs; everybody photographs their slides. Three pieces of physical evidence are usually enough.

The mount

Cardboard mounts with a heat-sealed edge belong mostly to the 1940s through the 1970s. Rigid plastic mounts appear in the 1970s and dominate the 1980s and 1990s. Glass mounts were sold to protect transparencies for projection and are common in the 1960s to 1980s — note that glass mounts trap dust and condensation and are the one mount worth replacing. Machine-printed mounts carrying the processing lab's name and a "PROCESSED" month and year are the easiest case of all: the date is printed on the object.

The edge printing

Along the film rebate, between the sprocket holes, the manufacturer printed the film name, an emulsion number and a frame number. "KODACHROME", "KODACOLOR", "SAFETY FILM", "ESTAR" and the process names ("K-14", "E-6", "C-41") all date the stock, sometimes to a decade. Film edge printing changed over the decades in ways that collector references and the manufacturer's own archives document.

The date stamp

From the late 1970s, many labs printed the processing date on the mount in red or orange. If it is there, and it usually is not, you are done.

If you want to place the image rather than the film — that is, work out when the picture was taken rather than when the roll was developed — the same logic applies as for prints: the format dates the object, and costume and faces are supporting evidence only.

Cold, dark and vertical — and not in the projector carousel.

Slide film and colour negatives are the two things in a family collection where cold storage genuinely changes the outcome. The rule of thumb from conservation practice: every 10°C drop in temperature roughly halves the rate of dye fading. A cupboard at 22°C and a refrigerator at 4°C are not the same material twenty years later.

1. Get them out of the projector carousel, the cardboard box, and the attic. The carousel is open to dust and light; the attic cycles through 40°C days and freezing nights, which is the worst possible environment for film.
2. Sleeve them in archival plastic — polypropylene, polyethylene, or polyester (Mylar). Never PVC. PVC pages from the 1970s and 1980s are still in albums everywhere, and they off-gas plasticiser that fogs and softens film and photographic paper. If a page smells of a new shower curtain, it is PVC.
3. Store vertically, in a cool dark place. A cupboard in an interior room, away from an outside wall, is fine for a decade. A frost-free refrigerator is better for a valuable colour collection; if you freeze film, seal it in a moisture barrier and let it come to room temperature sealed before opening, or condensation will do more damage than the warmth you avoided.
4. Do not project for hours. The projector lamp is a controlled dose of exactly what fades slide film. Ten minutes to see a holiday is nothing. A "slide evening" of three hours is a measurable loss.
5. Keep the negatives with the collection, not in the bin. The most common loss in family archives is not fading; it is the person who kept the prints and threw away the negatives. The negative is the original.

For the storage detail that applies to all photographic material — boxes, humidity, the specific rooms to avoid — the storage guide is the longer version. If the material is already wet, or smells strongly of vinegar and is sticking to itself, the emergency page takes priority over everything here.

Three ways to scan film, and what each is actually for.

The settings that matter

3,200 dpi on a 35mm frame yields roughly 3,000 by 4,500 pixels — enough to print a good A4 and to crop. Below 2,400 dpi you are keeping a thumbnail, not an original. Scan to TIFF, 16-bit if the software offers it, and keep that as the master; JPEG is for derivatives. Do the inversion and colour correction yourself or in the scanning software, but never save over the master.

Digital ICE, and the one film it breaks

Infrared dust and scratch removal (Digital ICE) works by scanning an infrared channel that the film's dyes pass through but dust does not. It fails on Kodachrome and on silver-based black-and-white film, because their structure blocks infrared the way dust does — the software then "smooths" real image detail. If the film is Kodachrome or black-and-white, turn ICE off and clean the film with a blower and a lint-free cloth instead.

The dust is the whole job

Almost all film-scanning complaints are about dust, and almost all dust is fixable before you press scan: a rocket blower, a clean room, and handling by the edges. Wet-cleaning fluids exist and are best avoided until you have practice, because a bad wet-clean removes emulsion.

How many there are decides the method, not how much you care.

A full carousel holds 80 slides. People usually have eight to twenty carousels, so somewhere between 600 and 1,600 frames, plus a shoebox of negatives nobody has counted. That is a different job from 60 prints, and it is why the arithmetic matters: at a minute and a half a frame on a film scanner, 1,600 slides is forty hours of work before any correction.

The scanning calculator takes your frame count, an hourly value and a service price, and returns the break-even. The default numbers are not in a service's favour: film scanning by hand is slow enough that a lab at $0.50–$1.50 per frame usually wins on anything above a few hundred frames, unless you are doing it in batches over months.

What to scan first, if you are not going to scan everything: the slides of people, not places. A photograph of a mountain in 1974 has no information in it that is not in a thousand other photographs. A photograph of your grandmother at a kitchen table in 1974 has information that exists nowhere else. The sorting guide applies to film as much as to prints — triage before you scan, always.

Once the files exist, the collection has moved from a chemical problem to a digital one, and the digital problem is easier: a master folder, a naming scheme, and two copies in two places. A scanned slide that lives only on one laptop is not preserved; it is just better organised.