What the stamp is
The box maker's certificate is a round or oval printed mark, traditionally on a bottom flap, in which the manufacturer certifies the construction of the board. It exists because carrier classification rules historically required it.
It typically carries some combination of: the manufacturer's identity, a burst strength or edge crush value, a size limit, and a gross weight limit.
The claim that matters: ECT
Edge Crush Test, in pounds per linear inch, is the best single predictor of stacking performance because stacking load travels through the vertical edges. Double-wall bulk board typically runs 1,000 to 1,300; triple wall 1,800 to 2,200; five wall up to 3,200.
If the certificate carries an ECT value and the board matches the construction you measured, you have a useful number.
The claim that is mostly historical: burst
Mullen burst strength measures puncture resistance and was the basis of older carrier rules. It still matters when contents are sharp, but it tells you very little about whether you can stack two loaded containers.
A certificate showing only a burst value on a bulk container is a sign the board was specified under an old rule set rather than for stacking performance.
Why the stamp is unreliable on used stock
On recovered containers, the certificate frequently describes a box that no longer exists. Units get trimmed, so the size limit is wrong. Bottom flaps get replaced, so the stamp may have come from a different container entirely. Some units carry two stamps from two lives.
This is not usually deception; it is just what happens to a stamp printed years ago on a container that has been through several plants and a knife table.
What to verify instead
Measure. A caliper at two points on a cut edge, or a folded flap if you do not have one, tells you the wall count directly.
- Double wall BC: about 9/32 inch (7.1 mm).
- Triple wall BCC: about 7/16 inch (11.1 mm).
- Five wall AAC: about 5/8 inch (15.9 mm).
- Count the flute layers visible at a cut edge — two for double wall, three for triple, five for five wall.
What to ask for, and why
On used stock, ask for the construction checked per unit and treat any strength figure as a range implied by that construction. On new production, ask for an actual certificate for the run.
Anyone offering you a single certified ECT number on recovered fiber is quoting a measurement from a different box on a different day, in a different humidity. The construction is checkable; the number is not.