How to Prevent Candles from Cracking While Cooling
Cracks and sinkholes on the surface of candles are one of the most common aesthetic and technical problems for makers and small manufacturers. Beyond their appearance, they indicate that the wax has contracted unevenly during cooling. In this article, you will learn why this happens, what steps to take to prevent it, and how to correct it if it has already appeared. At the end, we invite you to our in-person workshops in Barcelona to practice these techniques.
Why candles crack when cooling
- Thermal contraction: solid wax occupies less volume than liquid wax. If the surface cools and solidifies before the interior, the retreating core leaves voids or cracks.
- Inappropriate pouring temperature: pouring too hot or too cold causes poor adhesion between layers or greater shrinkage.
- Rapid cooling: drafts, cold surfaces, or sudden temperature changes create tension in the wax.
- Formulation and additives: certain blends, colorants, or high fragrance loads can increase contraction.
- Candle size and geometry: thick or tall-walled pieces retain heat longer, increasing the difference between the core and the surface.
Recommended materials and tools
- Suitable wax: choose the blend designed for the type of candle (pillar/mold, jar, etc.). Professional blends often include additives to reduce sinkholes.
- Reliable thermometer: essential for controlling melting and pouring temperatures.
- Molds or containers: it's best if you can preheat them (especially metal or thick silicone molds).
- Correct wicks: the wick influences the internal heat during the first burn and the final appearance.
- Draft-free space: a location with a stable temperature, protected from windows and fans.
Practical steps to prevent cracks
- Control melting and pouring temperatures
- Heat the wax according to the manufacturer's recommendations (many vegetable waxes melt at 70–80 °C). Allow it to cool to the specified pouring temperature (e.g., 55–65 °C for many pillar blends).
- Pouring too hot results in greater shrinkage; too cold can prevent adhesion between layers.
- Preheat molds and containers
- If the mold is cold, the surface solidifies instantly and the interior contracts, leaving voids. Slightly preheating (about 30–40 °C, depending on the material) reduces thermal shock.
- Pour in layers for thick or wide pieces
- For large diameters or deep molds, pour in two or more stages. Wait until the surface is stable (not completely solid) before the next layer: this improves adhesion and distributes contraction.
- Avoid drafts and rapid cooling
- Place candles in a draft-free location with a stable temperature during the first few hours. Slow and even cooling minimizes cracks.
- Use anti-sinkhole additives when necessary
- Many manufacturers offer specific additives for pillar or mold candles that reduce contraction. Use them according to the recommended proportion.
- Test colorants and fragrances in samples
- Some pigments and fragrance loads affect the structure. Conduct small tests before producing the final piece and respect the maximum recommended load.
What to do if a crack already appears
- Fill with hot wax: heat wax to the pouring temperature and fill the crack with a spoon or wax syringe. Allow to cool in a controlled manner.
- Smooth with controlled heat: a low-power hairdryer or heat gun at a distance can help the wax settle again—work carefully to avoid deforming the candle or burning yourself.
- Add a surface layer: pouring a thin, cold layer at a low temperature can level the surface.
- Integrate the crack into the design: in decorative candles, sometimes turning the texture into an intentional feature (varnish, contrasting color, or matte finish) is an aesthetic option.
Best practices for repeatable results
- Record: note melting and pouring temperatures, cooling times, formulations, and additive percentages for each batch.
- Test small: before scaling up a large piece, make test molds and adjust variables.
- Quality control: check pieces after 24–48 hours; some shrinkages take time to manifest.
Common mistakes and quick fixes
- Not using a thermometer: always measure. Sight does not replace thermal control.
- Forgetting to preheat molds: one minute of preheating prevents many problems.
- Overloading fragrance or color: reduce the load and retest.
- Exposing to drafts: protect candles during the first few hours.
Conclusion
Preventing candles from cracking while cooling is a matter of controlling temperature, gradual cooling, and choosing the right formulation. With systematic testing and some adjustments (preheating molds, pouring in layers, using additives), you will achieve smooth surfaces and professional finishes.
Want to practice this in person? At Vento Barcelona workshops, we teach you how to formulate, pour, and finish crack-free candles using vegan waxes and professional techniques. These are practical classes, for all levels, led by our instructors. Book your spot and learn everything about candlemaking: https://vento.barcelona/talleres
