Dear Dr. Vela:
I’ve started candle burning tests, but I don’t know how to do it. Which parameters should I measure?
The parameters that need to be measured are defined in the “standard F2417 from the American Society for Testing Materials” (ASTM F2417). Before starting any tests, and depending on your candle formulation, it is very important that the candles rest for a reasonable time:
- Vegetable waxes: 14 days
- Paraffin waxes: 24 – 48 hours
- Mixes: 1 day – 14 days
The testing room should have a neutral atmosphere, the temperature should be between 20 and 30 °C, avoiding wind drafts. There should be a 6 inches gap between candles to prevent heat transfer between them. These measures will provide more accurate candle burning parameters.
The length of the burn cycles will depend on the type of candles being tested:
-Tea-light or tea candles: get burned at once, until the candle extinguishes itself.
-Gel candles: should burn for 8 hours and then be allowed to reach room temperature or cool down for 3 hours before starting a new cycle.
-All other candles: burn at 4 hours cycles. Let them cool down for at least 3 hours before starting a new cycle or until they reach room temperature.
The ASTM “Candle Burn Performance Test,” requires the following procedure:
1. Prepare the test zone as described above, making sure to clearly mark each candle during the test.
2. Trim the wick to 1/4″ before each start cycle. Observe each burn cycle and record the following data after each hour:
a. The maximum height of the flame (from the bottom of the arch to the flame’s tip). This should be less than 3″ (or 3 3/4″ if the candle is designed for religious use).
b. The container is intact, not cracked or broken.
c. Soothing (black smoke) does emanate excessively from the wick.
d. The container has not damaged the surface on which it is sitting.
e. The candle has not been knocked over or spilled (this applies to pillars)
4. Record the flame height at least every 4 hours, even if you are “watching” it every hour.
5. If any of the requirements from step 3 are not met, consider the candle as a failure.
6. After the candle cools down, repeat steps 2 through 6 until the candle fails or gets extinguished.
If you want to measure the “Rate of Change ” (ROC), write down the candle’s initial weight and record the weight again after the final burn. Subtract the initial weight from the final weight and divide this value by the number of hours burned. This result is the total wax consumed per burning hour and the units are grams/hour.
Other parameters that can be measured, especially for container candles, are the diameter of wax pool (usually melts one inch per hour) and the depth of the wax pool (it should not be more than 1” per hour at the end of a 4 hour burn cycle).
Keep in mind that the burning performance tests should get done during the development stage, as part of your quality control you should also test every time the formula is changed, or when a new batch is produced; the number of samples depends on the lot size.