In the majority of cases a lumpy top on a soy candle is the result of pour temperature. Many people do not place enough importance on pour temperature, some not even using a thermometer.
If you want a great looking soy candle with smooth tops and good glass adhesion, investing in a good quality thermometer is a must.
A lot of people are caught by surprise, when after a couple of months of pouring, their candles all of a sudden develop holes or pits when they have changed nothing. They may not have changed anything but the weather may have changed. The season and your ambient temperature of the room you are pouring in, all play a big part in how your candle dries. You may use air conditioning in summer and a heater in winter. This will all affect your candles. On a particularly cold day you may also have to heat your glass or container slightly so it is warm to touch.
Lumpy tops can usually be fixed by increasing the pour temperature in increments until the problem disappears. The recommended pour temperature is a recommendation only and should be used as a guide for a starting point. If the recommended pour temp does not work you will have to experiment to find the correct temperature for your conditions and fragrance.
Fragrance and essential oils can also cause holes and lumpy tops. Fragrances like vanillas and essential oils such as lavenders and rose geranium are notorious for ‘curdling’ the top of soy candles. This can be overcome by pour temperature adjustment once again however if this does not work you may have to adjust the amount of FO or EO you are using.
It does come as a surprise to many but pour temperature is absolutely critical for your candles and may change with the seasons. Extensive testing and a detailed diary is a must for all candlemakers!