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D261-75 – Standard Test Method Technical Guide
ASTM D261 – 75 (Reapproved 1999) provides the authoritative specification for the dry pigment known commercially as Prussian blue, Chinese blue, Milori blue, or iron blue. This standard is essential for ensuring consistent quality, purity, and performance evaluation of this classic inorganic pigment across various industrial applications.
🧪 Composition and Chemical Requirements
The standard dictates a strict chemical identity. The pigment must be the direct reaction product of an iron salt solution with a ferrocyanide or ferricyanide solution. It must be entirely free from extenders and diluents such as barium sulfate, silica, silicates, calcium carbonate, calcium sulfate, and magnesium carbonate. A total of up to 5% of other substances may be added during manufacture solely for improving working properties or quality. Mandatory conformance limits are as follows:
| 🟦 Property | 📏 Specification Limit | 🎯 Primary Test Method |
|---|---|---|
| Total Matter Soluble in Water | 1.0%, max | ASTM D 1135 |
| Moisture Content | 8.0%, max | ASTM D 1135 |
| Organic Colors or Lakes | None permitted | ASTM D 1135 |
| Acidity/Alkalinity (as H₂SO₄ equivalent) | 0.1%, max | Methyl Orange Indicator |
| Extenders & Diluents (BaSO₄, Silica, CaCO₃, etc.) | Absent | ASTM D 1135 |
| Mass Color and Tinting Strength | Mutually agreed limits vs. standard | ASTM D 387 |
📐 Sampling Protocol and Test Methods
Representative sampling is critical. Two samples must be taken at random from different packages for each lot, batch, or day’s pack. For shipments exceeding 10,000 lb, two samples are required for each 5-ton unit of production. The purchaser has the option to test these samples individually or blend them into a composite sample.
| ⚖️ Production Unit or Shipment Size | 📐 Minimum Samples |
|---|---|
| Per Lot, Batch, or Day’s Pack | 2 |
| Shipments under 10,000 lb (4,536 kg) | 2 |
| Each 5 tons over 10,000 lb | 2 |
Chemical analysis is conducted per ASTM D 1135 (Chemical Analysis of Blue Pigments), covering identification, water-soluble matter (extraction or conductivity), and moisture (Brabender tester or toluene distillation). Physical performance tests for mass color and tinting strength are performed per ASTM D 387 using a mechanical muller.
⚡ Key Performance Characteristics and Quality Insights
The standard provides critical guidance on how to prioritize quality metrics.
💡 Critical Insight from Note 1: The physical properties and tests of iron blue, particularly the tinting strength, are explicitly considered a better measure of value than the percentages of chemical constituents. The mass color and character of the tint must match an agreed standard.
⚠️ Important Testing and Composition Rules: Any test procedures not covered by ASTM D 1135 or D 387 must be mutually agreed upon between purchaser and seller. The 5% allowance for added substances is strictly for working properties or quality improvements—it is not a license to add extending fillers.
This standard remains the definitive guide for confirming the identity and performance of iron blue pigments.
❓ Frequently Asked Questions
🔍 Which commercial names does this specification cover?
The specification covers the dry pigment sold as Prussian blue, Chinese blue