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D6409-99 – Standard Test Method Technical Guide
🎯 Scope and Terminology
ASTM D6409-99 (Reapproved 2020) establishes a standardized procedure for conducting color tests on all types of vegetable tanning extracts and raw vegetable tanning materials. The objective is to determine the specific color these materials will impart to a clear, clean hide or skin during the tanning process. The standard defines several essential terms for the procedure:
- Sheepskin Skiver: The grain split of a de-wooled sheepskin.
- Pickled Skiver: A sheepskin skiver preserved in a solution of salt and sulfuric acid, providing a standard substrate for testing.
- Vegetable Tannins: Natural astringent mixtures obtained from plant tissues that possess the chemical and physical properties required to convert hides into leather.
Values stated in SI units are the standard; values in parentheses are for information only. This practice was developed in accordance with internationally recognized principles on standardization established by the WTO TBT Committee.
⚙️ Apparatus, Reagents, and Reference Documents
The practice requires specific commercially available reagents and apparatus to guarantee consistent results. The pickled sheepskin skiver must be specially and consistently selected for the test. The following equipment and materials are specified:
| 🟦 Component | 📏 Specification |
|---|---|
| Pickled Sheepskin Skiver | Grain split from de-wooled sheepskin; pickled in salt and sulfuric acid |
| Borax | Commercial grade (tannery grade) |
| Acetic Acid | Glacial, commercial grade |
| Sodium Acetate | Crystal, commercial grade |
| Sodium Chloride | Crystal, commercial grade |
| Containers | 10 L and 3 L capacity; must be chemically resistant |
| Shake Bottles | 0.95 L (32 oz), approx 21.6 cm (8.5 in) overall height, with rubber stoppers |
The following ASTM standards and the ALCA method are directly referenced:
| 📐 Standard / Method | 🎯 Title / Application |
|---|---|
| ASTM D1517 | Terminology Relating to Leather |
| ASTM D4901 | Preparation of Solution of Liquid Vegetable Tannin Extracts |
| ASTM D4905 | Preparation of Solution of Solid, Pasty and Powdered Vegetable Tannin Extracts |
| ASTM D6405 | Extraction of Tannins from Raw and Spent Materials |
| ALCA A40 | Color Tests with Sheepskin Skiver (Original source method for D6409) |
📊 Test Procedure, Significance, and Use
According to the summary of practice, a specimen of the pickled skiver is tanned with a sample of the vegetable tanning liquor. This process allows the operator to determine the exact color which will be imparted to a standard hide or skin tanned with that specific liquor. The resulting color of the tanned leather specimen is then evaluated.
This practice provides a standard procedure for comparing the color of leather tanned with different tanning extracts or mixtures of extracts. It is also critically important for monitoring the color consistency of tannery liquors over time. Consistent color output is vital for quality control in leather finishing and production.
For accurate comparative results, the pickled sheepskin skiver must be specially and consistently selected for the color test. Variations in how the skiver is prepared can drastically alter color uptake.
As stated in Section 1.3 of the standard, users must establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to handling the chemicals used in this practice.
❓ Frequently Asked Questions
🔍 What is the primary purpose of ASTM D6409?
The practice covers making color tests of all types of vegetable tanning extracts and materials. It is specifically designed to determine the color that will be imparted to a tanned hide or skin.
💡 What is a “pickled sheepskin skiver”?
It is the grain split of a de-wooled sheepskin that has been preserved in a solution of salt and sulfuric acid. This prepared state provides a stable and highly reproducible substrate for the standardized color test.
⚡ What size shake bottles are required for the test?
The standard specifies shake bottles with a capacity of 0.95 L (32 oz) and an overall height of approximately 21.6 cm (8.5 in). These bottles must be fitted with rubber stoppers.
📌 How does this standard relate to ALCA Method A40?
ASTM D6409 was adapted from ALCA Method A40, “Color Tests with Sheepskin Skiver,” and serves as its direct replacement. The ALCA method is explicitly listed as a referenced document.