Acid-Free vs Buffered Archival Boxes: pH and Lignin Specs

"Acid-free" printed on a carton is a statement about pH on the day the board was tested. It says nothing about lignin, nothing about how much alkaline reserve is left to absorb acids later, and nothing about whether the board suits photographs. Those are separate measurements, and an archival spec should name each one.

Records cartons in the custom banker boxes product page range can be built on standard corrugated or on archival board. The table below shows what each grade actually measures and the figures suppliers and standards quote.

Acid-Free vs Lignin-Free vs Buffered vs Unbuffered: The Numbers

Board grade	What is measured	Figures commonly quoted	Usually chosen for	Watch for
Standard recycled corrugated	Usually nothing archival	No stated pH, alkaline reserve or lignin limit	Transitory and fixed-retention business records	Recycled and unbleached fibre; no permanence claim should be made
Acid-free (pH only)	pH of the board	pH 7 or above; permanent-paper standards set 7.5 to 10	Medium-term paper storage	Without a reserve, pH can fall as the board absorbs acids from air and contents
Lignin-free	Lignin content, expressed as a kappa number	NISO Z39.48: lignin no more than 1%, kappa no more than 7. ISO 9706: kappa no more than 5	Long-retention paper records	"Lignin-free" with no kappa figure is a label, not a test result
Buffered (alkaline reserve)	Calcium carbonate held in the board	NISO: at least 2%. ISO 9706: 20 g/kg (the same 2%). Archival corrugated is often quoted above 3%, at pH 8.0 to 9.5	Permanent paper files, manuscripts, bound records	The alkaline environment may not suit some photographic processes or protein fibres
Unbuffered, lignin-free	Near-neutral pH, no added reserve	No reserve figure by design	Some photographs and textiles, on a conservator's advice	Less protection against acids that migrate in from outside
Gray boxboard, metal-edge	Board caliper plus the archival tests above	One supplier lists 60 pt board, acid-free, lignin-free and buffered	Flat documents and lighter collections	Thinner than corrugated; not meant for heavy stacked loads

The pattern is simple. pH tells you where the board starts. Alkaline reserve tells you how long it can hold that pH. Kappa tells you how much lignin is there to break down into acids over time.

What Each Measurement Means in Practice

pH

pH runs from 0 to 14, with 7 as neutral. Archival corrugated is usually tested by cold extraction (TAPPI T 509 is one method suppliers cite) and quoted in a range such as 8.0 to 9.5. A single pH figure with no method is hard to compare.

Alkaline reserve, or buffer

The reserve is calcium carbonate added during papermaking. Conservators often call it the buffer. It neutralizes acids from the air, from handling and from acidic folders stored inside. It is consumed over time, so a buffered carton is not acid-free forever.

Kappa number

Kappa measures how much lignin remains in the pulp (TAPPI T 236 is a common test). Archival vendors quote kappa figures as low as 1 to 2 for bleached alpha-cellulose board. Ordinary kraft corrugated is made from unbleached fibre and is not quoted this way.

Reading the Standards Without Overclaiming

Several standards are named in archival specs. They cover different things, so ask which one a board has actually been tested against.

ISO 9706: a permanent-paper standard setting pH 7.5 to 10, an alkaline reserve of 20 g/kg and kappa of 5 or below.
ANSI/NISO Z39.48: the US permanent-paper standard, with pH 7.5 to 10 for uncoated paper, a reserve of at least 2% and lignin of no more than 1%.
ISO 16245: covers boxes made from cellulosic board for paper and parchment. Type A board must meet ISO 9706-style requirements, kappa included. Type B drops the kappa limit. Tests listed include pH, reserve, kappa, colourfastness, optical brighteners and water absorption (Cobb 60).
Photographic Activity Test (PAT): checks whether an enclosure material reacts with photographs. NEDCC cites ISO 14523 for it.

A board that has been tested to one of these can be described that way when the supplier provides the test data. A board that has not been tested should be described by its measured values only.

Buffered or Unbuffered? Matching Board to the Contents

Paper records, letters and bound volumes: buffered, lignin-free board is the usual choice.
Photographs: NEDCC advises that the chemistry of the enclosure and a passed PAT result matter more than pH alone. For colour prints, cyanotypes and albumen prints, it does not recommend buffered or unbuffered as a blanket rule.
Wool, silk and other protein fibres: unbuffered board is commonly suggested. Confirm with a conservator for the specific item.
Mixed collections: house photos and textiles in their own inner enclosures, then use the buffered outer carton for the bulk paper.

Archival Box Styles and Construction Options

Lift-off lid on B-flute archival corrugated: the bulk-records format. One supplier lists starch adhesive at pH 7 and no optical brighteners on this style.
Drop-front boxes: the front panel folds down, so photographs and flat items can be slid out without lifting the stack. NEDCC suggests this style for photos stored flat.
Metal-edge boxes: metal reinforces the corners of thinner gray boxboard.
Handholds: optional. Archival suppliers offer the same box with and without cut-outs, because an open handhold lets dust and light in.
Labels and print: print with water-based ink on the exterior only, and keep any label adhesive off the inside surfaces.

How to Check a Supplier's Archival Claim

Ask for substrate documentation rather than relying on the word printed on the carton. A useful data sheet lists:

pH, with the test method.
Alkaline reserve as a percentage of calcium carbonate.
Kappa number or lignin percentage.
Fibre type, such as bleached alpha cellulose or recycled.
Adhesive type and whether optical brighteners are present.
PAT result, if the box will hold photographs.

Some public archives write the marking into their own carton spec. The Library of Virginia, for example, treats a carton as archival when it is acid-free material with "Acid Free" or a pH above 8 printed on the outside.

The buyers who usually need this level of documentation are museums, special collections libraries, university archives, and the teams that hold permanent corporate and government files.

Mistakes to Avoid With Acid-Free Archival Boxes

Reading "acid-free" as permanent. Without a reserve, a board that tests neutral today can turn acidic in storage.
Buffering everything. Putting photographs or silk in buffered board without checking can cause the very damage you are trying to avoid.
Undoing the carton with what goes inside it. Acidic folders, rubber bands and rusting clips inside an archival carton cancel much of its benefit.
Ignoring the room. Heat and humidity speed up chemical decay faster than board grade can slow it.
Paying for archival board on short-retention files. Records due for destruction in a few years rarely need it. Spend on better labeling or stronger construction instead.

Spec an Archival Run

Tell The Printing World what the cartons will hold (paper, photographs, textiles or mixed), the retention period, the board data you need on file and the quantity. We will confirm the board and its documentation before proofing. Email sales@theprintingworld.com or message +16133831487.

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Reorders on locked artwork may reduce proofing time because the structure and artwork are already approved, but standard production remains 10–14 business days after artwork approval unless otherwise confirmed.

Acid-Free vs Lignin-Free vs Buffered Archival Boxes: pH, Reserve and Kappa Compared