**Cool the aluminum molds from the outside, and let the cubes cool throughout before loading them.** A firm surface can conceal a warm, soft center.

For your batch of 72 cubes, these are the practical options:

| Technique | How to use it |
|---|---|
| **Fan and cool metal surface** | Space the molds apart on a substantial aluminum or steel plate, and blow air across them. This is the easiest starting method. |
| **Water-cooled molds** | If the molds are watertight, stand them securely in a shallow tray of cool water, keeping the water below their rims. Circulate or replace the water as it warms. Keep every drop out of the sugar. |
| **Brief refrigeration** | Refrigeration can accelerate cooling. Remove the molds before the candy becomes cold enough to collect condensation in the room. The Culinary Institute of America specifically cautions against chilling molded sugar excessively. [CIA guidance](https://www.ciafoodies.com/using-candy-molds/) |

**Expect cooling to take tens of minutes.** As rough planning estimates, allow **30–60 minutes in still room air**, or **15–30 minutes with active cooling**. These are estimates, not measured times for your molds; starting temperature, mold thickness, spacing, and cooling contact could put your batch outside those ranges.

Make a trial cube first. Ideally, put a temperature probe into its center during casting and time how long it takes to approach room temperature. Check again with the full batch: 72 hot cubes can warm a cooling plate or water bath substantially.

For supporting the cake:

- Put a **rigid cake board** between the cake and cubes to distribute the load.
- Test cooled cubes under the intended load for the intended display duration.
- Keep them dry and protect them from moist cake or icing. Humidity control matters for hard candy; industrial cooling uses cool, dry air. [ASHRAE guidance](https://handbook.ashrae.org/Handbooks/R18/SI/R18_Ch42/r18_ch42_si.aspx)

If you dissolve the white sugar in water before cooking, take it to **hard crack, about 300–310°F / 149–154°C at sea level**. The remaining water content affects whether it becomes hard at room temperature. [Exploratorium](https://annex.exploratorium.edu/cooking/candy/hardcrack.html)