Buyers compare freeze points on the front label and rarely ask about the boiling point. In a cold market that habit costs little. In a hot market it is the single most expensive number to ignore — because boil-over, not freezing, is the failure that strands drivers in summer traffic. The good news: both numbers come from the same dial. The percentage of ethylene glycol in the mix sets the freeze point and the boiling point together, so once you know the grade, you know roughly what the coolant should boil at — and you can use that to catch watered-down product before it reaches a customer.
The physics in one paragraph
Pure water boils at 100°C and freezes at 0°C. Pure ethylene glycol boils at about 197°C. Mix them, and the blend sits between: more glycol means a lower freeze point and a higher boiling point, moving in opposite directions from water. That is the entire basis of coolant grades. A -25°C grade carries roughly 40% glycol; a -35°C grade roughly 50%; a -45°C grade roughly 60%. The glycol also carries the corrosion inhibitors into every corner of the system, which is why full-dose glycol matters beyond the two numbers.
| Glycol by volume | Freeze point (approx.) | Open boil point (approx.) | Typical market grade |
|---|---|---|---|
| 0% (water) | 0°C | 100°C | reference only |
| 30% | -16°C | 103–104°C | -15°C grade |
| 40% | -25°C | 104–105°C | -25°C grade |
| 50% | -37°C | 106–107°C | -35°C grade |
| 60% | -52°C | 108–110°C | -45°C grade |
One more layer: a radiator in a working car is pressurised. A typical 1.1 bar cap adds roughly 3–5°C to the effective boiling point, which is why a 50% mix that boils at 106–107°C in an open pot keeps an engine cool past 110°C in the vehicle. It is also why a worn cap that no longer holds pressure shows up first as summer boil-over, even with good coolant inside.
Why a -35°C label proves nothing by itself
Ethylene glycol is not the only way to hit -35°C on a refractometer. Methanol, cheaper alcohols and dissolved salts also depress the freeze point — and they are the backbone of the low-end market. The catch is what they do to the other end of the scale: a methanol-water blend can read -35°C on the freeze side yet boil at 102–103°C, barely above water. Methanol also evaporates on every heat cycle, so the freeze protection quietly disappears within months. Salt-based blends corrode solder and cast iron. Both are why two drums marked -35°C can behave completely differently in a Lagos or Cairo summer.
Three field tests that expose watered-down coolant
- 1Boil test: heat a sample in an open vessel with a thermometer. A genuine -35°C glycol coolant reads 105°C or higher; a methanol-water fake struggles to reach 103°C. No lab needed — a kitchen thermometer and a cup work.
- 2Refractometer on two dates: test a sealed sample when the drum arrives and again after 8–12 weeks of storage. Glycol holds its reading; methanol drifts upward in freeze point as the alcohol evaporates.
- 3pH and reserve alkalinity: a cheated blend has almost no inhibitor package, so its reserve alkalinity is near zero. A pH strip plus an RA titration (ASTM D1121 method) separates 'protective coolant' from 'coloured water' in ten minutes.
How the grades map to real markets
- 0°C tropical grade: lowest glycol, best heat transfer, and the full inhibitor package is what actually protects the engine — boil-over protection comes from glycol plus a sound pressure cap.
- -25°C / -35°C grades: the export workhorses. 104–107°C open boil points when built on full-dose glycol; demand the boil number on the test report.
- -45°C and below: highest glycol and the highest boiling point, for genuinely cold markets. Watch pour point and storage temperature before winter shipping.
See BOLITCH coolant grades
Every BOLITCH grade is built on full-dose ethylene glycol with a measured boiling point — our range runs 101–107°C across the line, and the number is on the test report, not just the label. See the grade list at bolitch.com or request the spec sheet and MSDS (available on request).