Reference Tables: Wax Densities, Melting Points, and Fluid Properties
Wax and Fluid Density at Calibration Temperature
The tables below consolidate measured and manufacturer-reported density values for the wax compounds and carrier fluids most commonly found in Mathmos lamps. All density figures are given at 25 °C unless otherwise noted, as this represents the closest practical ambient approximation to the wax’s neutral-buoyancy window — the narrow thermal band in which correct lava motion depends on a density differential of no more than approximately 0.002 g/cm³ between wax and fluid.
| Component | Density at 25 °C (g/cm³) | Notes |
|---|---|---|
| Paraffin wax (standard grade) | 0.900–0.915 | Varies with carbon chain length |
| Microcrystalline wax (high-melt) | 0.920–0.940 | Higher branching, denser solid phase |
| Carbon tetrachloride (historical) | 1.594 | No longer used in production units |
| Water/surfactant carrier (modern) | 0.997–1.002 | Surfactant content shifts value slightly |
| Translucent wax blend (Mathmos-typical) | 0.912–0.926 | Compound-dependent; see calibration notes |
The spread within the paraffin category is not trivial. A wax sitting at 0.900 g/cm³ and a fluid adjusted to 0.998 g/cm³ gives a differential of 0.098 g/cm³ — far too wide for the wax to achieve neutral buoyancy at operating temperature. The density calibration page covers the practical methods for narrowing this gap through additive adjustment.
Melting Point Ranges for Common Wax Constituents
Melting point is not a fixed value for blended wax compounds; it is a range, bounded by the solidus (the temperature at which melting begins) and the liquidus (the temperature at which the compound is fully molten). The width of this range determines how gradually the wax transitions between states and therefore how smoothly it rises and falls.
| Wax Component | Solidus (°C) | Liquidus (°C) | Melt Range Width |
|---|---|---|---|
| Paraffin (low molecular weight) | 46 | 52 | 6 °C |
| Paraffin (high molecular weight) | 56 | 63 | 7 °C |
| Microcrystalline wax | 60 | 75 | 15 °C |
| Stearic acid additive | 67 | 69 | 2 °C |
| Typical Mathmos production blend | 58 | 66 | ~8 °C |
A narrow melt range, as exhibited by stearic acid, produces sharp phase transitions that can cause the wax to behave abruptly — rising quickly and cohering into a single mass rather than separating into smaller blobs. A wider range, characteristic of microcrystalline wax, smooths the transition considerably. This accounts for why microcrystalline fractions are blended into production compounds even though they raise the overall density and require compensatory fluid adjustment.
Thermal Expansion Coefficients
Thermal expansion determines how much the wax’s density decreases as it heats toward its melt range. Because density drives buoyancy, the expansion coefficient is the mechanism that makes motion possible in the first place. Values below are volumetric expansion coefficients in the solid phase.
| Material | Volumetric Expansion Coefficient (×10⁻⁴ per °C) |
|---|---|
| Paraffin wax (solid) | 3.5–4.5 |
| Microcrystalline wax (solid) | 2.8–3.6 |
| Water at 25 °C | 2.57 |
| Water/glycol fluid blend | 3.1–3.9 |
The implication here is that wax expands faster than pure water across this temperature range, which is precisely what allows it to cross from denser-than-fluid to less-dense-than-fluid as the lamp warms. A water/glycol carrier blend, expanding at a comparable rate to the wax, narrows the window still further — which is why glycol concentration requires careful control.
For the underlying physics governing these values, the thermal behaviour page provides a full treatment of melting, expansion, and convection. Readers working through a specific failure diagnosis will find these tables cross-referenced throughout the wax failure causes and cures pages, where density and melting point data are applied directly to real fault conditions.