Overview - ISO 5940-1:2019 (ring-and-ball method)
ISO 5940-1:2019 specifies the ring-and-ball method for determining the softening point (softening temperature) of pitch for electrodes used in the production of aluminium. The method is applicable to pitches with softening points equal to or greater than 30 °C and describes apparatus, sample preparation, test procedure, calculation, precision and reporting requirements.
Key topics and technical requirements
- Scope: Applies to carbonaceous pitch used in aluminium electrode manufacture; softening points ≥ 30 °C.
- Principle: A moulded disc of pitch in a tapered brass ring is heated in a liquid bath (water or glycerol) with a steel ball placed on the surface. The softening point is the temperature at which the pitch sags to touch a lower support under the ball’s mass.
- Apparatus: Detailed ring-and-ball assembly:
- Brass tapered rings (depth 6.4 mm; internal diameters 15.9 mm/17.5 mm) and steel balls (9.53 mm diameter, 3.50 g ± 0.05 g).
- Bath beaker (≈800 ml), ring support, ball centering guides, tripod, burner and draught screen.
- Thermometric devices (PT100, thermistor, thermocouple or compliant mercury thermometers) with specified ranges and accuracy. Devices must indicate temperature to 0.5 °C (for 0–120 °C range) or 1.0 °C (for higher ranges) and be stable for the test.
- Heating medium:
- Water for softening points 30 °C to 85 °C.
- Glycerol for softening points > 85 °C. (Note: glycerol can give slightly higher values around 85 °C.)
- Procedure highlights: Melt and homogenize ~50 g laboratory sample (Annex B), mould two rings from a single melt, heat bath at 5 °C/min, record temperature when pitch around each ball contacts lower support. Reject results if paired values differ >1 °C.
- Precision: Repeatability and reproducibility given:
- <85 °C: repeatability 0.5 °C; reproducibility 2.5 °C.
-
85 °C: repeatability 1.5 °C; reproducibility 3.0 °C.
- Reporting: Sample ID, reference to ISO 5940-1:2019, heating medium used, measured result (mean of two readings to nearest 0.2 °C), thermometric device, deviations and observations, date.
Practical applications and users
- Quality control and acceptance testing of electrode pitch in aluminium smelters and carbonaceous materials manufacturers.
- Independent test laboratories performing physical property testing of pitch.
- R&D and product development teams validating pitch formulations for electrode baking and performance.
- Procurement and compliance teams requiring standardized softening point data for supplier specifications.
Related standards
- ISO 6257 - sampling of pitch for electrodes (referenced normative document).
- Other parts of the ISO 5940 series (see ISO website for related test methods in carbonaceous materials for aluminium).
Keywords: ISO 5940-1:2019, ring-and-ball method, softening point, pitch for electrodes, aluminium production, pitch softening temperature, testing procedure, thermometric device, glycerol, water, repeatability, reproducibility.
Open preview (PDF)Carbonaceous materials for the production of aluminium — Pitch for electrodes — Part 1: Determination of softening point by the ring-and-ball method
