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D5583-06 – Standard Test Method Technical Guide
ASTM D5583-06 describes a non-chemical, biological procedure for estimating the retention of wood preservatives using the black mold Aspergillus niger. The method provides a visual index of preservative content at discrete locations, requiring only standard microbiological equipment and a controlled incubation environment.
🔬 Scope and Significance of the Aspergillus Bioassay
This test method is designed to assay wood for approximate preservative content without composite sampling. It is applicable to any preservative of uniform composition that can diffuse into the culture medium. The method is especially useful for monitoring non‑pressure treated millwork, assessing longitudinal penetration via end‑grain sampling, and observing preservative losses during service. As noted in the standard (Note 1), with simple adaptation it can also be applied to fungicidal paints and glues.
⚠️ Limitation: This test method is not intended for comparing different preservatives nor for estimating retentions of preservatives with variable composition, such as creosote. It can, however, be used to compare the relative potencies of such preservatives.
⚙️ Test Apparatus and Controlled Environment
The procedure requires conventional equipment for fungal culture and aseptic transfer, along with a tightly controlled temperature range for incubation.
| 🟦 Equipment Category | 🔬 Specification |
|---|---|
| Sterilization | Autoclave for preparing nutrient agar and sterilizing tools. |
| Incubation Chamber | Room or chamber controlled at 22–27°C (72–82°F). |
| Culture Vessels | Glass or plastic Petri dishes (100 × 15 mm); 250 mL Erlenmeyer flasks. |
| Transfer Tools | Transfer needle and gas or alcohol burner for aseptic technique. |
| Stock Culture Tubes | Test tubes (150 × 22 mm) for maintaining pure Aspergillus niger stock. |
| Storage | Refrigerator for stock culture preservation. |
💡 Technical Note: Aseptic precautions are not strictly required during the sample assay itself, but are essential for maintaining a pure stock culture of Aspergillus niger. Contaminated cultures can lead to invalid test results.
📊 Test Procedure and Data Interpretation
Small wood specimens of prescribed size are placed onto nutrient agar in a Petri dish freshly seeded with Aspergillus niger spores. The culture is incubated for three to four days. The key measurement is the “zone of effect”—the white area around the sample where the fungus cannot produce its typical black spores due to the inhibitory action of the diffusible preservative.
The size of this zone serves as the index of preservative retention. A reference relation is established for each preservative by assaying specimens containing a known gradient of retentions (e.g., pentachlorophenol in Fig. 3 of the standard).
| 📐 Parameter | ⚡ Specification / Observation |
|---|---|
| Test Organism | Aspergillus niger (black mold) |
| Specimen Dimensions | Prescribed size (detailed in the full standard) |
| Incubation Period | 3 to 4 days |
| Incubation Temperature | 22 to 27°C (72 to 82°F) |
| Positive Indication | White zone (zone of effect) surrounding the specimen |
| Result Index | Size of the zone of effect |
❓ Frequently Asked Questions
🔍 What types of preservatives can be evaluated with this method?
This test is for preservatives of uniform composition that can diffuse into the agar medium. It is not designed for variable mixtures like creosote, though it can assess their relative potency. Pentachlorophenol is a typical example given in the standard.
💡 Why is a white zone (zone of effect) observed around treated samples?
The preservative diffuses from the wood into the surrounding nutrient agar. At high enough concentrations, it prevents Aspergillus niger from sporulating, resulting in a visible white halo against the black spore lawn. The diameter of this zone correlates with preservative retention.
⚡ What are the critical environmental controls for this test?
The incubation temperature must be strictly maintained at 22 to 27°C (72 to 82°F). Lower temperatures will slow fungal growth, while higher temperatures may stress the culture. The incubation period is fixed at three to four days.
📌 Can this method be used for materials other than solid wood?
Yes. The standard notes (Note 1) that with appropriate, simple adaptation, the method can be applied to other products such as fungicidal paints and glues. The assaying can be done wherever convenient, as strict asepsis is only needed for stock culture maintenance.