11.2 Darkroom Facilities, Processing, and Artifact Control

ASNT's radiographic general exam treats Darkroom Facilities, Techniques and Processing as official RT topic 2. The heading is not a request to recite every brand of developer. It asks whether you can keep a latent image honest from the cassette to the viewer: a light-tight room, a safelight that does not fog the film, a wet sequence whose time and temperature actually match the film system's class, and the judgment to tell a processor artifact from a part discontinuity.

ASTM E94, ASTM E1742, and ASME Section V, Article 2 assume a controlled process behind every density and IQI number in Section 11.1. Those documents are industry and code references, not ASNT exam publications. Film data sheets and the employer's procedure name the chemistry, the developer time-temperature chart, the processor replenishment rates, and the safelight filter. If the procedure tightens a tank temperature or forbids a filter, the procedure wins.

A Class I exposure that is then fogged, underdeveloped, or printed with a roller streak is not a valid radiograph. Topic 2 punishes anyone who treats the darkroom as housekeeping.

Darkroom facilities

The room has one job: let you handle unprocessed film without adding density that the part did not earn.

Light-tightness. Door seals, a light lock or revolving door, and a pass box that cannot be open on both sides at once are the usual architecture. White-light leaks print as edge fog, streaks from the door, or a general rise in base-plus-fog. Cassette leaks (cracked lids, tired foam, loose latches) do the same thing in the bay and then travel into the darkroom as "mysterious" fog.

Dry bench and wet bench. Load and unload cassettes, cut film, and store boxes on the dry side. Tanks, processor feed, and wash plumbing stay on the wet side. Crossing those benches with wet gloves is how fixer splash reaches undeveloped film and prints a chemical streak through the weld.

Cleanliness and climate. Dust on the worktop becomes plus-density spots. Grit on a lead screen becomes a repeating cassette mark. Humidity that is too low invites static; humidity that is too high invites sticking, water marks, and fungus on stored film. Follow the film manufacturer's storage and handling sheet rather than inventing an unpublished ASNT humidity number. Temperature swings between a hot bay and a cold tank room are how reticulation (a leather-like emulsion crackle) gets started when film then hits a hot dryer.

Safelight. Industrial orthochromatic and most industrial x-ray films are commonly handled under a GBX-2 (or procedure-named equivalent) filter, at the bulb wattage and minimum distance the filter and film data sheets allow, for no longer than the handling time those sheets assume. A cracked filter, a brighter bulb "so I can see," a safelight hung over the feed tray, or a ten-minute chat with unprocessed film on the bench is a fog setup. Prove the installation with a coin test (or the filter manufacturer's test): unexposed film on the bench, an opaque coin or paper clip on the emulsion, safelight on for the maximum handling time, then process. If the coin's outline is visible, the safelight is unsafe. Safelight fog is usually a uniform or edge-wise density rise. It flattens contrast and can push a legal 1.8 ROI into an unreadable band once you add the real exposure.

Store film on edge, in the original packaging, away from process radiation, heat, and chemical fumes. Expired or heat-fogged film already has a head start on base-plus-fog before the safelight ever comes on.

The wet sequence: developer, stop, fixer, wash, dry

Classic manual processing is five stages. Automatic processors compress the same chemistry into a roller path. Skipping a stage does not "save time"; it changes the image.

Developer is an alkaline reducing solution (commonly a hydroquinone pair with Metol or a Phenidone-type agent, plus preservative, accelerator, and restrainer — the data sheet names the mix). Activity is a time-and-temperature pair. A typical manufacturer chart still anchors manual tanks near 68 °F (20 °C) and a stated number of minutes; warmer tanks need less time, colder tanks need more. That pairing is a manufacturer/procedure fact, not an ASNT secret. A shop that "develops until it looks right" has abandoned the film-system class that Section 11.1 just used to justify the IQI.

Underdevelopment looks like underexposure: low density, low contrast, IQI holes that fade. Overdevelopment plus fog looks like a dirty safelight or aged film. You separate those causes with a control strip processed with the production film, not by guessing at the viewer.

Stop bath is usually a dilute acetic acid, or a vigorous water rinse where the procedure allows it. The point is to neutralize carry-over developer so fixation starts cleanly.

Fixer (sodium or ammonium thiosulfate plus hardener) must clear the film — the moment the milky halide disappears — and then continue for the multiple of clearing time the data sheet names (twice the clearing time is common photographic teaching; use the printed chart). Incomplete fixing leaves halide that later stains and that can print as a veiling density.

Wash is not a courtesy dip. Residual fixer attacks the silver image over weeks and months. Wash time, water flow, and temperature are process variables. A hypo-clearing agent, when the procedure uses one, shortens wash; it does not replace it.

Dry in a filtered cabinet. Water drops that dry on the emulsion become water spots — circular plus- or minus-density disks that candidates love to call porosity. Excess heat plus a cold incoming film is how emulsion reticulates.

Automatic processors add replenishment rates, roller cleanliness, and transport speed to the same chemistry story. If the control strip's speed index, contrast index, or base-plus-fog is out of the procedure's band, production radiographs processed on that machine are suspect until the tank is restored.

Temperature, time, and process control

Developer activity rises quickly with temperature. The written exam does not need a fake Q10 number from you; it needs the idea that a tank 5 °F hot and a tank 5 °F cold are different processes, and that you do not "make up" a hot tank by guessing a shorter dip unless the manufacturer's chart says so.

Run sensitometric control strips (a pre-exposed step tablet) at the interval the procedure names. Record base-plus-fog, a speed step, and a contrast difference. Drift toward fog is chemistry, safelight, or film storage. Drift toward thin speed is exhausted developer or a cold tank. Contrast collapse is often developer oxidation or contamination from fixer.

Manual hangers that drip into the next film, a fixer-contaminated developer tank, and a processor that has not been replenished all produce chemical streaks — irregular plus-density trails that do not follow a weld pass.

Artifacts: the handling and processor family

Topic 2 expects you to name the common marks and to know they are not automatically discontinuities.

Static looks biological and almost never follows a fusion line. Pi lines repeat at a mechanical pitch that has nothing to do with weld beads. Crimps are crescents that match a thumb, not a toe crack. Water spots are round, but they sit in the film plane — they do not obey weld geometry when you compare two shots.

Artifact versus discontinuity

The Level II question is always the same: is this mark in the part?

Use this ladder:

1. Look at the signature. Tree-like static, a crescent crimp, roller-pitch pi lines, a fingerprint, a circular water disk, or a splash that starts at the film edge is processing or handling until proven otherwise.
2. Is it even on the part image? Marks in the clear border, under the identification letters only, or in a region of film that never saw the weld are not weld discontinuities.
3. Double-load or second film. If two films were exposed together and the mark is on only one, it formed after the beam left the part (processing, handling, or a single-emulsion scratch).
4. Change the cassette or screens. Screen dirt and a damaged cassette repeat with that hardware. A new cassette that is otherwise identical is a cheap experiment.
5. Reshoot. If the mark moves relative to the part, changes shape, or vanishes, it was not a discontinuity in the metal. If it stays locked to the same location on the part across independent exposures and independent cassettes, treat it as being in the part and evaluate it under Section 11.3.

Do not grind a weld because a pi line crossed it. Do not accept a linear root indication because "we get a lot of processor marks." The reshoot and the handling geometry decide the family; the acceptance table decides the disposition only after the mark is a real discontinuity.

Safelight fog is the one artifact that is not local. It is a contrast tax on the whole sheet. The coin test and the control strip catch it. Turning up the viewer does not.

Digital: ghosting, dead pixels, gain/offset

CR and DDA skip the wet tanks and invent their own topic-2 artifacts. The classification rule does not change: if it is not in the part, it is not a defect in the part.

Ghosting (lag). A previous exposure remains faintly visible on the next image. On CR this is often incomplete erasure of the imaging plate. On a DDA it is residual charge or scintillator afterglow, especially after a high-dose shot of thick steel followed by a thin aluminum part. The ghost does not belong to the current part. Erase the plate fully, allow the panel to recover, and acquire a new offset if the procedure requires it. A ghosted IQI from the last job is not today's sensitivity proof.

Dead or bad pixels. Individual diodes or clusters that read stuck black, stuck white, or noisy. A maintained bad-pixel map interpolates isolated pixels. A cluster in the area of interest can hide or fake a pore and is often rejectable as an image, not as a part, until the map and the procedure say the cluster is allowed. Do not interpret a line of dead pixels as lack of fusion.

Gain and offset (flat-field) errors. A DDA is corrected with a dark (offset) frame and a flood (gain) frame so that pixel-to-pixel sensitivity disappears. Dust on the scintillator, a changed collimator, a different energy, or a stale correction leaves a fixed-pattern blotch that sits in detector coordinates, not in part coordinates. Move the part or the panel: a gain artifact stays put on the detector; a slag line stays put on the weld.

Related digital handling marks: scratched or cracked CR plates, dirt in the scanner, saturation (clipped white or black) from a gross over- or under-exposure, and tiling or stitch lines on multi-panel setups. None of these is solved by a stronger display window.

Realistic exam scenarios

Unprocessed film left under a bright safelight for a long conversation, then processed, shows a coin-test outline and a high base-plus-fog. The IQI hole that was marginal at 1.9 is gone. This is safelight fog, not a sudden loss of subject contrast in the weld. Fix the safelight; reshoot.

A repeating set of faint lines crosses several welds at the same spacing on every film from one automatic processor. The spacing matches a roller. These are pi lines. Clean or replace the roller; do not report "aligned incomplete fusion" on four pipes that share a tank.

A crescent sits in the parent metal, away from any stress raiser, and is absent on the second film of a double load. That is a crimp, not a crack. A fine linear dark indication that stays in the weld toe on two independent exposures is not excused as a crimp.

A DDA frame of a thin flange, taken immediately after a thick nozzle shot, shows a ghost of the nozzle IQI. Ghosting. Erase or recover; do not evaluate the ghost as tungsten.

Topic 2 language is short: facilities that do not add density, a wet or digital process that is inside its control band, and a mark that is classified as handling or as metal before anyone talks about accept or reject.