Reading a bottle

Why this is worth learning

Grade disputes almost always come down to bottle appearance, and almost always because two people are looking at the same wall and describing different phenomena. A stained bottle and a chalked bottle look similarly unattractive and mean entirely different things: one is cosmetic and the other means the polymer has degraded.

You need a torch and about ten seconds per container. Shine it through the wall from outside and look at the transmitted light, then look at the same spot in raking light across the surface. Those two views separate all four defects.

Haze

What it looks like: the wall transmits light but you cannot see a sharp shadow through it. Even, all over, slightly milky.

What it is: fine surface scratching plus a thin film, usually from repeated filling and emptying with something mildly abrasive or from a very light residue. Sometimes it is simply age — HDPE surfaces microscopically roughen with thermal cycling.

What it means: essentially nothing structural. A hazed bottle is B grade rather than A because you cannot see the level clearly and because it usually accompanies a service history, not because the container is weaker. Haze does not propagate and does not lead anywhere.

Stain

What it looks like: colour, usually in patches or in a tide line, sometimes following the pattern of how the container drained.

What it is: pigment or oxidised product bonded to and slightly into the surface layer. Dark fruit concentrates, some dyes, iron-bearing solutions and a great many industrial products do this. It is not dirt and it will not wash out — we have run stained bottles through L4 with steam and they come out identically stained and completely clean.

What it means: cosmetic, with one important practical consequence. A stained container will tint a light-coloured product. That is a specification issue rather than a defect: if your product is white and the container is brown inside, you have bought the wrong grade. If your product is a dark degreaser, the stain is irrelevant and you should not pay to avoid it.

Chalk

What it looks like: a dry, powdery, matte white bloom on the outside surface, heaviest on the top and on whichever face pointed south. Rub it with a thumb and some of it comes off on your thumb.

What it is: ultraviolet degradation. UV photons break polymer chains at the surface; the fragments migrate and crystallise as a powder. The bloom is literally the container shedding degraded material.

What it means: this one is structural and it is the defect people most often dismiss. Chalked HDPE has lost impact strength in the affected layer, and the affected layer thickens with continued exposure. A heavily chalked container will crack from a knock that a sound one would shrug off, and it will do so at the top radius where the exposure was worst.

Light chalk on the top surface only: C grade, fine for water and ballast. Chalk on the sidewalls, or chalk heavy enough to leave a clear mark on your thumb: we granulate it. There is no repair and no wash that reverses it.

Stress-whitening

What it looks like: a bright white line or patch, sharply bounded, usually along a fold, around a dent, or radiating from a point of impact. In transmitted light it is opaque where the surrounding material is translucent.

What it is: crazing. The polymer has been strained past its elastic limit and micro-voids have opened between the crystalline regions. Those voids scatter light, which is why it goes white. It is the physical signature of permanent damage.

What it means: this is the fatal one. A stress-whitened line is a crack that has not finished happening yet. It will propagate under load, and hydrostatic pressure from a full container is exactly the load it responds to. We reject on this without discussion, at any grade, at any price.

Haze you ignore. Stain you exploit. Chalk you price. Whitening you refuse.

The ten-second version