Three Surface Problems: Sweating, Frosting and Wet Spots
The candle comes out of the mould, and two days later there are droplets of oil on its surface. Or the surface is covered in white, crystalline patches. Or wet-looking areas have formed on the inside of the glass where the wax has pulled away from it. Three problems, three separate causes; all three look like a "faulty candle" to the customer.
Sweating is the fragrance oil rising to the surface of the candle; oil the wax cannot hold seeps out. Frosting is the white, crystalline appearance that forms on the surface of natural waxes (soy in particular); it is a change in the crystal structure of the wax, not a contamination. Wet spots are the areas where the wax has shrunk and pulled away from the glass container; they cause no scent or burning problem, they are an appearance issue.
All three relate to how the candle cools, and they are mostly solved through production parameters: fragrance oil ratio, addition temperature, stirring, pour temperature and cooling rate.
Sweating: Why It Happens, How to Prevent It
Wax dissolves the fragrance oil in its oil phase and holds it within its crystal structure as it solidifies. The holding capacity varies by wax type: paraffin generally 8–10%, soy 6–10%, coconut blends higher, beeswax low. When the capacity is exceeded, or the fragrance oil does not bind well to the structure of the wax, the excess comes out.
| Cause | Symptom | Correction |
|---|---|---|
| Fragrance oil ratio above the wax's capacity | Droplets on the surface within 1–3 days across the whole batch | Lower the ratio; do not exceed the maximum load given by the wax supplier |
| Fragrance oil added at too low a temperature | Scattered; present in some candles, absent in others | Add the fragrance oil at the supplier's recommended temperature (60–70°C for most waxes) and stir for 1–2 minutes |
| Insufficient stirring | Sweating in one area of the candle, none in another; uneven scent | After adding the fragrance oil, stir for at least 2 minutes, right down to the bottom of the vessel |
| Temperature cycling (warehouse, shipping, shop window) | Fine on the shelf, sweating in summer or in a sunlit window | Do not exceed 25°C in storage and shipping; keep away from window light |
| Fragrance oil incompatible with the wax (too many oil-based components) | Sweating even at a low ratio | Choose a fragrance oil designed for candles; run a base compatibility test |
| Very slow cooling (large candle in a warm room) | In large candles, not in small ones | Cool at room temperature, in an environment free of draughts but not warm |
The most practical test for diagnosing sweating is to pour the same wax at three fragrance oil ratios (6%, 8% and 10%, say) and wait a week. Whichever ratio sweating starts at, the wax's real capacity is the one below it; the number that counts is the one you measured, not the one the supplier quoted.
Frosting: The Crystal Problem of Natural Wax
Soy and other vegetable waxes can take on different crystal forms as they cool; as they shift over time to the more stable form, white, snow-like patches form on the surface. This is called frosting. Paraffin does not frost; in soy wax it is unavoidable to a degree and is accepted as "the mark of a natural wax". It can still be controlled.
- Lower the pour temperature
Pour soy wax at the lower end of the supplier's recommended range (50–55°C for most soy). Wax poured at a high temperature cools slowly and frosts more.
- Warm the container
Warm the glass container before pouring (warm air or a warm water bath). Cold glass chills the wax touching it suddenly; both frosting and wet spots increase.
- Cool slowly and evenly
Cool the candles away from draughts, at room temperature, placed close together. Rapid cooling increases frosting; do not use a fridge or an open window.
- Reduce the colourant
Frosting is far more visible in dark-coloured soy wax. Uncoloured or pale wax shows the same frosting less.
- Consider additives
Some wax blends and additives (coconut-soy blends, a small proportion of stearic acid, or the manufacturer's anti-frosting additive) regulate the crystal structure. Changing an additive also changes burning and scent throw; test it.
- Keep storage constant
Temperature fluctuation triggers frosting. A constant temperature from production to shelf is the most effective measure.
Wet Spots and the Fragrance Load Balance
Wet spots form when the wax shrinks as it cools and pulls away from the glass; seen through the glass, they leave areas that look wet and oily. The measures are the same as for frosting: warm the container, cool slowly, adjust the pour temperature. Preventing them entirely is difficult; frosted or coloured glass hides the spots.
What the three problems have in common is their relationship with fragrance load. As the fragrance oil ratio rises, the risk of sweating rises, frosting increases in some waxes, and wet spots change as the fragrance oil softens the wax. A high fragrance load looks attractive with its promise of a strong hot throw; but a load beyond the wax's capacity does not increase the scent, it pushes it to the surface. The right ratio for a candle is not "the highest" but "the highest the wax holds", and it is determined by the sweating test.
The way to increase hot throw is not more fragrance oil but a more suitable one: a fragrance oil designed for candles, with its volatility balanced to the burning temperature, gives a stronger throw at 6% than an incompatible fragrance oil does at 10%. Ask the fragrance oil supplier for oils tested in candles and the recommended load; then confirm in your own wax.
A production record is the only way to trace the source of these three problems: wax lot, fragrance oil lot and ratio, fragrance oil addition temperature, pour temperature, room temperature, container type. The only difference between a sweating batch and a good one is very often a single line in this record.
Frequently Asked Questions
The most common questions about candle surface problems.
There are oil droplets on the surface of the candle, can it be used?
At what temperature should I add the fragrance oil?
My soy candle has turned white, is it faulty?
How do I tell sweating from frosting?
What is the maximum fragrance oil ratio?
How do I prevent wet spots in glass containers?
My candles sweat in summer but not in winter; why?
Can I increase the fragrance oil ratio for a stronger scent?
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