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D3991-94 – Standard Test Method Technical Guide
📐 Scope and Significance
ASTM D3991-94 (Reapproved 2012) establishes standardized specifications for classifying the fineness grade of wool or mohair. It is applicable to fiber in the grease, pulled, washed, or scoured state, as well as card sliver and materials processed beyond carding on the woolen system. The standard specifically excludes worsted system materials (which involve a combing process and are covered under D3992) and mixed lots requiring segregation before classification.
Grade assignment relies primarily on two methods. Visual examination is widely accepted in commercial trade as defined by the Official Standards of the United States for Grades of Wool (Section 31.03). However, in cases of dispute, the standard mandates a referee procedure utilizing the measured average fiber diameter and fiber diameter standard deviation as established by Test Method D2130 (Microprojection).
📊 Fineness Grade Requirements
The specifications define distinct grade boundaries for wool (Table 1) and mohair (Table 2). For wool, grades correspond directly to traditional blood-type and numeric categories, with specific upper and lower limits on average fiber diameter and a maximum allowable standard deviation to control variability. For mohair, the grades are defined solely by the average fiber diameter range.
🟢 Key Referee Criterion: Per Section 5.2, the average fiber diameter and the standard deviation must both fall within the specified ranges for the grade to be definitively assigned. This dual metric prevents classification based on mean alone.
| 🟦 Grade / Blood | 📏 Numeric Grade | 🎯 Avg. Diameter (µm) | ⚡ Max. Std Dev (µm) |
|---|---|---|---|
| Fine | 80s | ≤ 17.70 | 4.09 |
| 1/2 Blood | 70s | 17.71 – 19.15 | 4.59 |
| 3/8 Blood | 64s | 19.16 – 20.59 | 5.00 |
| 1/4 Blood | 62s | 20.60 – 22.04 | 5.31 |
| Low 1/4 Blood | 60s | 22.05 – 23.94 | 5.64 |
| Common | 58s | 23.95 – 25.64 | 6.09 |
| Braid | 56s | 25.65 – 27.84 | 6.79 |
| 🟦 Grade | 🎯 Avg. Diameter (µm) |
|---|---|
| Kid | ≤ 24.0 |
| Young Goat | 24.1 – 28.0 |
| Adult | 28.1 – 32.0 |
| Strong Adult | 32.1 – 36.0 |
⚙️ Test Methodology and Referee Protocol
Compliance with the grade specifications is verified using Test Method D2130, the standard test method for diameter of wool and other animal fibers by microprojection. This method involves aligning, mounting, and measuring individual fibers using a calibrated microscope. The number of fibers measured must meet the minimum count required by the method to ensure statistical significance for both the mean and standard deviation calculations.
⚠️ Application Limitation: Per Section 5.3, these specifications are not intended for mixed lots. If a lot consists of fleeces of different grades that are to be segregated into smaller, more homogeneous lots before use, D3991 should only be applied after segregation is complete.
❓ Frequently Asked Questions
🔍 How does D3991 differ from D3992?
D3991 covers raw fiber forms (grease, scoured, card sliver) and woolen-system materials. D3992 specifically governs the fineness of wool top and mohair top, which are combed products of the worsted system. Related species like alpaca are covered under ASTM D2252.
💡 What constitutes the “referee procedure” when visual grading is disputed?
When a visual grade is in dispute, the mandatory referee procedure requires an objective measurement of the average fiber diameter and standard deviation following the Test Method D2130 (Microprojection) protocol. The results are compared directly against the specifications in Table 1 or Table 2 of this standard.
⚡ Why is the “Standard Deviation” parameter critical in the wool grading table?
The standard deviation quantifies fiber diameter variability. Two batches can have the same average micron value but differ drastically in uniformity. A high SD negatively impacts spinning performance and yarn evenness. D3991 controls both the mean and the SD to preserve processing quality.
📌 What does the ε1 (2012) annotation signify in the standard designation?
An epsilon (ε) superscript indicates an editorial correction. The note accompanying the 2012 reapproval states that the terminology section was updated to maintain consistency with Terminology D4845 (Relating to Wool) and D123 (Relating to Textiles).