The Preservation Index: Understanding Archival Quality

Demystifying the science of preservation requires understanding key archival metrics that determine material longevity and suitability for conservation work. This guide provides detailed explanations of critical factors that conservators evaluate when selecting restoration materials.

pH Level: The Acid-Free Foundation

pH level measures the acidity or alkalinity of a material on a scale from 0 to 14, with 7 being neutral. For archival materials, a pH between 7.0 and 8.5 is considered ideal. Materials with pH below 7.0 are acidic and will deteriorate over time, causing yellowing, brittleness, and eventual disintegration.

Ideal Range
pH 7.0 - 8.5
Optimal

Neutral to slightly alkaline, ensures long-term stability

Acceptable Range
pH 6.5 - 7.0
Acceptable

Slightly acidic but may require buffering

Warning Range
pH 5.0 - 6.5
Caution

Acidic, will cause deterioration over time

Unacceptable
pH < 5.0
Unsafe

Highly acidic, unsuitable for archival use

Lignin Content: The Hidden Deterioration Factor

Lignin is a natural polymer found in wood pulp that causes paper to become acidic and brittle over time. Archival-quality paper must contain less than 1% lignin. Lignin-free or low-lignin papers are manufactured using cotton, linen, or chemical pulping processes that remove this damaging component.

Archival Standard
< 1%
Lignin-Free
Standard Paper
15% - 30%
High Lignin

Alkaline Reserve: Future Protection

Alkaline reserve refers to the buffering capacity of a material—its ability to neutralize acids that may form over time. Materials with 2% to 3% alkaline reserve (typically calcium carbonate) can counteract acidic compounds that develop from environmental exposure or internal chemical reactions.

Optimal Reserve
2% - 3%
Buffered

Provides long-term protection against acid formation

Minimal Reserve
1% - 2%
Partially Buffered

Some protection but may require monitoring

No Reserve
0%
Unbuffered

Vulnerable to future acidification

Permanence & Durability Standards

International standards provide objective criteria for evaluating material permanence and durability. ISO 9706 (permanence) and ANSI/NISO Z39.48 (permanence of paper) establish testing protocols and minimum requirements for archival materials used in conservation work.

International Standards Overview

ISO 9706
Specifies requirements for permanent paper used in publications and documents intended for long-term use. Requires pH 7.5-10.0, alkaline reserve ≥0.4%, and lignin-free content.
ANSI/NISO Z39.48
American standard for permanent paper. Requires pH 7.5-10.0, alkaline reserve ≥2%, minimum tear resistance, and fold endurance specifications.
ISO 18902
Standard for imaging materials including photographic paper and storage enclosures. Specifies pH requirements and chemical stability for archival storage.

Visual Scoring System

Use this color-coded system to quickly assess material quality based on preservation metrics:

Green Zone
Ideal

pH 7.0-8.5, <1% lignin, 2-3% alkaline reserve. Suitable for long-term archival use.

Yellow Zone
Acceptable

pH 6.5-7.0, minimal lignin, some buffering. Requires careful monitoring.

Red Zone
Unacceptable

pH <6.5, high lignin, no buffering. Not suitable for archival applications.