Reading Stress Crazing: Field Notes From the Grading Pad
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Crazing starts at the four lower bottle corners, where hydrostatic stress is highest. Light it with a torch at a low angle rather than head-on. Fine frost across a corner radius predicts a weep within a year; discrete long hairlines running up from the base are more urgent than that.
Four months into training a grader, there is a moment where they stop seeing scuffs and start seeing crazing. It is the most important thing that happens in the job and it cannot be taught from a checklist, because the difference between the two is about pattern rather than appearance.
This is my attempt to write down what I look for.
What crazing is
Environmental stress cracking is what happens when a polymer under tensile stress meets a chemical that is mildly aggressive to it. Neither alone would do much. Together they open up a network of very fine cracks that propagate slowly, over months, from the surface inward.
In an IBC the stress is hydrostatic. A filled bottle pushes outward hardest at the bottom, and the bottom corners are where the wall has to turn a radius while carrying that load. So that is where it starts, essentially always: the four lower corners, in the radius, within six inches of the base.
The chemistry can be almost anything mildly polar — a surfactant, a dilute acid, a glycol with an additive package, the wrong cleaning agent used once in a previous life. Which is why you cannot predict crazing from prior contents alone. You have to look.
Lighting, which is most of the skill
Crazing is invisible head-on in daylight. It is obvious with a torch held at roughly 20 degrees to the wall, shining across the surface rather than into it, with your eye close to the same plane. What you are looking for is light scattering off crack faces that are perpendicular to the beam.
Practical sequence: kneel at a corner, torch low and raking across the radius, move the beam slowly up and down the first six inches. Then do the other three corners. Twenty seconds each. If you are standing up and shining a torch at the bottle, you will not see it, and you will grade a crazed bottle as Grade A.
Four patterns worth distinguishing
Fine frost across the radius
A dense, even haze of very short hairlines following the curve of the corner. The classic early pattern, and the most common. It means the mechanism is active and the bottle will weep, in our experience within about a year of returning to service under load.
Grade: not Grade A. Technical grade at best, and we would not put it under anything we could not afford to lose.
Discrete long hairlines running up from the base
Fewer cracks, each much longer, running vertically from near the base seam upward. This is further along and more urgent than the frost pattern: the cracks have coalesced and are propagating. We route these to rebottling or fabrication rather than selling them as containers.
Crazing around the discharge boss
A rosette around the outlet. Almost always mechanical rather than chemical: somebody over-torqued a fitting and put the boss root into tension. Unrepairable. This bottle goes to fabrication or regrind, and the cage goes on.
Surface chalking with no crack network
Powdery white on the sun-facing side that comes off on your thumb, but no hairlines when you light it properly. This is UV degradation, not stress cracking, and the distinction matters: chalking alone is survivable for a low-stress application like a planter or a non-pressurised catchment tank, and it is not survivable in a tote that will be stacked.
What people mistake for crazing
- Mould flow lines. Straight, regular, present from manufacture, usually running the same direction on every unit of the same model. Harmless.
- Cage chafe marks. Shallow abraded bands where a loose bottle has rubbed the lattice. A wear question, not a cracking one — but it tells you the bottle is loose in the frame, which is its own problem.
- Scuffing from handling. Random, directional, confined to the surface, no scatter under a raking light.
- Dye ghosting. Discolouration, not geometry. Entirely cosmetic.
Why the air test does not replace this
A 3 psi air-hold test tells you the bottle does not leak today. Crazing tells you whether it will leak in eight months. They answer different questions, and a crazed bottle passes an air test routinely — the cracks have not gone through the wall yet.
That is the whole reason we grade by hand rather than just running everything across the test rig. The rig is a snapshot; the corners are a forecast.
758 words · published 11 July 2023 by Dale Prichard, Senior grader at IBC Tote Market LLC. We correct posts rather than quietly deleting them; if something here is wrong, tell us and we will say what changed.