Method of Making Silicon Iron Compounds (James A. Parsons, No. 1,819,380)
This 1931 patent by James A. Parsons, a pioneering African American metallurgist and researcher for The Duriron Company, details a scientific breakthrough in the production of acid-resisting alloys. Parsons developed a high-speed method to stabilize the silicon content in iron, which was notoriously difficult to control but essential for industrial chemical equipment.
1. The Engineering Challenge: The “Brittleness vs. Corrosion” Balance
In the 1930s, silicon-iron alloys were the gold standard for handling corrosive acids. However, the window for a successful “melt” was incredibly narrow:
- The Target: Silicon content must be between 14.25% and 14.50%.
- The Risk of High Silicon: If the content exceeds 14.50%, the metal becomes too brittle, causing industrial parts to shatter under stress.
- The Risk of Low Silicon: If it drops below 14.25%, the alloy loses its acid-resisting properties and dissolves in chemicals.
Previously, manufacturers had to melt, cool, and chemically analyze the iron—a slow process that often led to inconsistent batches.
2. The Innovation: Resistivity as a Proxy for Chemistry
Parsons discovered a precise correlation between the electrical resistance of the molten metal and its silicon percentage. This allowed for “real-time” testing while the furnace was still running.
- The Test Bar: A sample is taken from the melt (between 2450°F and 2550°F) and cast into a specific chilled metal mold to create a bar 1/2 inch in diameter and 6 inches long.
- Kelvin Bridge Testing: Using a Kelvin bridge (a specialized instrument for measuring very low resistances), the bar’s resistivity is measured.
- 20 micro-ohms/cu. in. = 14.25% Silicon
- 25 micro-ohms/cu. in. = 14.70% Silicon
- 30 micro-ohms/cu. in. = 15.00% Silicon
Parsons established a resistivity curve (graph) that proved so accurate it rarely varied more than 0.1\% from a full laboratory chemical analysis.
3. The “High-Side” Strategy
Parsons’ process changed the order of operations to improve the physical quality of the castings:
- Initial Melt: The iron and ferrosilicon are mixed so the silicon is intentionally higher than the target (e.g., 15.5%).
- Analysis: The resistivity test is performed within 15 minutes.
- Correction: Instead of adding silicon (which causes “boiling” and makes the metal porous or full of air bubbles), Parsons added pure iron to “dilute” the silicon down to exactly 14.35%.
- Pouring: By the time the correction is made, the furnace has reached the optimal pouring temperature (2800°F to 2900°F).
4. Technical Advantages Summary
| Feature | Advantage |
| Speed | Determination in 15 minutes vs. hours for chemical analysis. |
| Precision | Accuracy within 0.1\% of the desired silicon content. |
| Structural Integrity | Avoids “boiling” by adding iron instead of silicon, resulting in denser, stronger castings. |
| Insensitivity | Carbon and Manganese levels do not interfere with the resistivity readings. |
Legacy of James A. Parsons
James Parsons was the first African American to hold a high-ranking position at Duriron and eventually earned several patents in metallurgy. His work was vital for the chemical industry during WWII, as his “Duriron” alloys were essential for the plants producing munitions and refining chemicals.
