Micro-cracks under raking light

The technique

Everybody inspects a bottle by shining a torch through it and looking at the transmitted light. That finds staining, haze and gross damage, and it is the right first move. What it cannot find is surface cracking, because a crack a few thousandths of an inch deep does not meaningfully change light transmission.

Instead: hold the torch almost flat against the outside of the bottle, so the beam grazes the surface at five or ten degrees, and look across the illuminated area rather than at it. Surface features throw shadows. Cracks become visible as fine dark lines, usually in a network, usually running in a preferred direction.

It takes about ten seconds per face once you have the angle. Most people have never done it, and the first time is genuinely startling on an old outdoor container.

What you are looking at

Environmental stress cracking. HDPE under sustained tensile stress — and a filled container's wall is permanently under tensile stress from hydrostatic pressure — will crack when that stress combines with an aggressive environment. UV exposure is one such environment; certain surfactants are another, which is why detergent containers sometimes show it early.

Why it matters more than chalking

Chalking is the visible symptom of UV degradation and it is what everybody grades on. Micro-cracking is the structural consequence of the same process and it is what actually causes failures. The two correlate but not perfectly: we regularly see containers with light chalking and substantial cracking, and occasionally heavy chalking with a sound substrate.

Grading on chalking alone therefore passes some containers that should not be filled and rejects some that are fine. Adding the raking-light check to our nine-point inspection in early 2025 moved roughly 3 % of containers between grade bands, in both directions.

What it means for buyers

1. Do the check on any container that has visibly lived outdoors, whoever you are buying from. A torch and ten seconds a face.
2. A cracked container is not automatically waste. Cracking under about five thousandths of an inch is fine for static, ground-level, non-transport water storage — irrigation, ballast, dust suppression.
3. It is not fine for anything that will be filled and moved. Transport vibration plus hydrostatic load is exactly the combination that turns a crack network into a split.
4. It is not fine for anything stacked. The bottom container in a stack sees the highest sustained stress.
5. Shade prevents essentially all of this. The difference between a covered and an uncovered container at five years is the difference between B grade and granulation.

If you have an outdoor array and you have never looked at it this way, go and do it this week. It is the cheapest inspection in this trade and it is the one that finds the failure before it happens rather than afterwards.