Grade 37 and grade 40 are both office-generated, both free of groundwood as their core requirement, and both sold into deinking furnishes. Buyers new to the grades often treat them as the same material at two price points. They are not. The definitions diverge on one property, and everything else follows from it.
What each definition covers
Sorted Office Paper (SOP, grade 37) is paper as typically generated by offices, containing primarily white and coloured groundwood-free paper and free of unbleached fibre. The specification allows a small percentage of groundwood computer printout and facsimile paper.
Sorted White Ledger (SWL, grade 40) is uncoated, printed or unprinted sheets, shavings, guillotined books and cuttings of white groundwood-free ledger, bond, writing and other paper of similar fibre and filler content.
Read them side by side and the difference is one word. SOP says white and coloured. SWL says white.
| Grade | ISRI No. | Colour | Prohibitives | Outthrows + prohibitives |
|---|---|---|---|---|
| SOP | #37 | White and coloured | 1% | 5% |
| SWL | #40 | White only | 0.5% | 2% |
Tolerances as published in ISRI PS-2013. The SWL limits are half and two-fifths of the SOP limits respectively — a reasonable proxy for the difference in sorting effort behind them.
Why colour is the dividing line
A deinking plant is in the business of removing ink and recovering brightness. Ink on white stock comes off. Dye in the sheet does not — it is part of the fibre furnish, not a surface contaminant. A bale with a meaningful share of coloured stock delivers lower final brightness than a bale of white stock of identical weight, whatever the deinking line does to it.
So the two grades are not really two qualities of the same thing. They are two different inputs:
- SWL is bought where final brightness is the constraint — printing and writing grades, tissue at the higher end.
- SOP is bought where brightness targets are more forgiving, or where the mill has the capacity to handle the colour load in the furnish.
What the tolerance gap is actually paying for
Going from 1% to 0.5% on prohibitives, and from 5% to 2% on the combined figure, is not a minor tightening. It means a second pass at the sorting line and a narrower set of accepted inputs at the point of collection.
The practical consequence is availability. SOP comes from a broad office collection stream and moves in volume. SWL depends on streams that are already predominantly white — converting operations, guillotine trim, printers' waste — plus the sorting to finish the job. A mill that needs a steady monthly SWL position should contract it as a position rather than expect to buy it spot.
"Office paper" is not a grade.
It is a description that covers both of these and several others. A contract written against it has no enforceable brightness expectation and no tolerance figures. Write #37 or #40.
The unbleached fibre clause
SOP's definition specifies free of unbleached fibre, and this is worth reading carefully because it is a different kind of requirement from the colour allowance. Unbleached kraft — brown envelopes, file folders with kraft backing, corrugated fragments that found their way into an office bin — is not a brightness problem that can be diluted. It is a contaminant in a deinking furnish.
In an office collection stream, brown stock is exactly what tends to arrive uninvited. It is one of the more common reasons a lot presented as SOP fails inspection.
How to specify either one
- State the grade number. #37 or #40, not "office paper" or "white ledger".
- State both tolerance figures on the offer, as published or as tightened by agreement.
- If brightness is the binding constraint, say so and contract #40. Buying #37 and hoping for a white-heavy delivery is not a specification.
- Agree what happens on a marginal lot before booking — allowance, rejection or re-sort. That conversation is far cheaper before the container sails.
We quote both grades, with the grade number and both tolerance figures stated on every offer. The full table is on the waste paper page, and the two-limit system is explained in OCC vs DSOCC.