Candle making

Scenting candles: fragrance load, flash point and cure time

Most scent problems come down to three numbers: how much fragrance you added, what temperature you added it at, and how long you waited before deciding it had failed. Get those right and an ordinary wax will smell good; get them wrong and no fragrance oil will rescue the candle.

Fragrance load is a percentage of wax weight

Fragrance load is a percentage of the weight of the wax, not of the finished candle or the volume of the container. The usual working range is 6-10%, and every wax has a maximum that the supplier publishes on its data sheet, commonly 8-10% for container soy. That figure is the ceiling, not the target.

The arithmetic is boring: 500 g of wax at 8% needs 40 g of oil. Weigh both on the same scale, and weigh the oil rather than measuring it by volume, because oils differ in density and a millilitre is not a gram.

Start a new fragrance at 6-8% rather than at the maximum. You cannot un-add oil, and an overloaded test candle misleads you about the wick as well as the scent.

Why more is not stronger

Wax holds only so much oil in solution. Past that point extra oil does not make the candle stronger, it simply stops being part of the wax:

• Sweating. Beads or a slick of oil on the surface, especially in a warm room.
• Pooling and seepage. Oil separating out and collecting at the bottom or against the glass.
• A worse burn. Excess oil clogs the wick and drowns the flame, causing flickering, sooting and self-extinguishing.
• A genuine fire risk. Free oil on top of a candle is fuel outside the wick, and it can flare.

If a candle at 10% smells weaker than the same candle at 8%, you passed the wax's limit and the burn has degraded.

Temperature, flash point and incorporating the oil

The addition temperature

Add fragrance at the temperature your wax manufacturer specifies — for most container soy somewhere around 80-85 °C (176-185 °F), with other waxes having their own figures. The reason is binding: the wax has to be hot and fluid enough for the oil to dissolve into it rather than sit as a separate phase. Add it too cool and the oil never fully incorporates, which shows up later as sweating and a weak hot throw.

Flash point

The flash point of a fragrance oil is the temperature at which its vapours can momentarily ignite near a flame. It is mainly a handling and shipping classification, worth knowing for storage and for working near an open flame. Whether adding oil above its flash point burns off the top notes is argued about far more confidently than the evidence supports; what is not in dispute is that your wax's recommended addition temperature is a tested figure.

Stirring

Stir for a full two minutes after adding the oil, and time it, because nobody stirs for two minutes by feel. Stir slowly, scraping the sides and bottom of the jug rather than whipping air in. Under-stirring is a common cause of a candle that smells fine in one part of the jar and of nothing in another.

Cold throw and hot throw

Cold throw is what you smell from an unlit candle: oil evaporating off the surface, so it depends mostly on surface area and how much oil sits near the top. Hot throw is what fills the room when the candle burns — oil vaporising off a warm pool of liquid wax and carried on convection.

They are different mechanisms, which is why a candle can be magnificent in the tin and silent when lit. Strong cold throw with no hot throw usually means the melt pool is too small because the wick is under-sized, the oil is not properly incorporated, or the fragrance does not perform in that wax. Test in that order: the wick is cheapest to check and likeliest; the wicking article covers how. Judge hot throw by walking into the room, since you go nose-blind to your own candles within minutes.

Cure time

Curing is the wax finishing its crystallisation and the fragrance binding into it. Soy commonly needs one to two weeks, with two as the safe default; paraffin is usually faster, often a few days; coconut blends one to two weeks. Cure at room temperature, lids on, out of direct sunlight.

This is why testing a candle the day after pouring tells you almost nothing. A soy candle at 24 hours often has a poor hot throw and then transforms after ten days. Judge a fragrance or a wick on a one-day-old candle and you will bin recipes that were fine. Pour a test batch, label it with the date, and leave it alone.

Essential oils versus fragrance oils

Essential oils are distilled or pressed plant extracts. Candle-grade fragrance oils are blends formulated to survive being dissolved in hot wax and vaporised off a melt pool. For candles the fragrance oils win, and it is worth being clear why.

Most essential oil constituents are highly volatile — that is what makes them smell — so they evaporate during the melt and the cure instead of staying in the wax. Many have low flash points, citrus especially, well below normal wax working temperatures. They are also chemically fragile, so what reaches the room is not what went into the jug. The usual result is a pleasant cold throw, a very faint hot throw, and several times the cost.

Heavier materials do better: patchouli, cedarwood, vetiver and the resinous oils are more robust than citrus or delicate florals. If you use essential oils, use those, expect a subtle candle, and do not raise the load to compensate — the wax's maximum does not change because the oil is natural.

Safety

Melt wax in a double boiler or a purpose-made melter rather than over direct heat, and never leave it heating unattended: wax that gets hot enough will ignite. Use a thermometer.

Never leave a burning candle unattended, and never burn one within reach of children or pets. Keep candles away from draughts, curtains and anything flammable, and burn them on a heat-resistant surface. Do not burn for more than about four hours at a stretch, trim the wick to about 6 mm (a quarter inch), and stop when roughly 1 cm of wax is left. Ventilate while you work, and store fragrance oils closed and out of sunlight.

The wax comparison covers how much of the throw is decided by the wax before fragrance enters the picture.