The fragrance oil is checked very carefully by law to ensure that it does not contain banned substances. Extensive testing is carried out on all of these fragrances before they are passed by governement scientific bodies as safe to use. Guidelines are produced which will inform you how much of the fragrance is recommended, that is, what is the maximum percentage that should be used.
Now... consider the essential oils - yes they are natural but not everything that is natural is good for you... the only guidelines that exist for essential oils are produced by aromatherapists and their governing bodies. For the purpose of the law they fall under the category of 'fragrance' or 'perfume' but have not undergone the same rigorous testing that the synthetic fragrance has. Consider this also... each batch of essential oil is different from the last, this is the way Mother Nature works, there is no consistency.
This is how the magic of aromatherapy works - the synergy of chemical constituents that we are unable to reproduce in the laboratory. But do you really need the Aromatherapy angle?
What I mean is.. the soap will wash off your skin and disappear down the drain. You are waterproof and will not under any circumstances absorb the soap into your body during your bath or shower - you will breathe in the scent, however if you have made cold or hot process soap there is no guarantee that any of the beneficial 'aromatherapy' constituents of the essential oil will have survived the rigorous soap making procedure. Therefor there may be no therapeutic value in the essential oil beyond its pleasant smell.
For this reason I always advise my students to choose oils for their scent rather than for therapeutic purposes. This may make you think twice about whether to use a synthetic soap fragrance or not, especially when there are so many wonderful synthetic soap fragrances to choose from. After all, you can't get a strawberry essential oil or a lily of the valley - yet these were certainly my best sellers, and my customers were fairly well educated in the synthetic vs natural debate. You may also find that the citrus essential oils lose their scent very quickly in cold process soap and the only way to get a good lemon or lime smelling soap is to use a synthetic fragrance.
If you wish to truly make a difference in your skincare you must choose the base oils in your soap very carefully.
Working with a soap fragrance can sometimes pose problems for your soap mixture. Especially if you are using the cold process method of soap making. Fragrance is always the last thing you add to the soap mix before pouring it into the mould. the synthetic fragrance can react with the mixture and produce a 'runaway trace' effect which ends up with the soap setting or 'seizing' in the pot before you have a chance to get it into the mould. Even if you managed to mix the soap fragrance without the mixture seizing, once you come to cut it, sometimes the fragrance is apparent as white or pale coloured streaks in the final bar of soap. Streaks or striations like this are caused by the fragrance reacting with the soap before it has been fully incorporated into the mixture.
Not all fragrances produce this runaway trace effect and some suppliers test them first and give advice on those which are likely to be tricky and require very quick working.
There are a a few ways of managing soap fragrances to get the best results:
Be aware that fragrances that do not react in small batches of soap may produce effects when used in larger quantities of mixture - mostly I think this is due to the length of time it takes to fully incorporate the fragrance into a larger batch.
The fragrance can be diluted by mixing with a quantity of oil taken from the recipe at the beginning.
You can increase the amount of water in the soap mixture (between 5 and 10 percent increase) in order to make the mixture more fluid.
Do not allow the soap to go to full trace before adding the fragrance - but make sure that the mixture IS DEFINITELY at a light trace or you could be left with a separated mess in the mould the following day. This is why most newbie soap makers stick to essential oils for the first few batches. Judging trace can take experience.
ALWAYS ensure that you have mixed the fragrance really well into the soap before pouring into the mould - but do not mix for too long or you risk seizing in the pot. Again, it takes the skill of the experienced soapmaker to manage this successfully. The danger in not mixing enough is that the fragrance may produce white striated lines in the soap or worse... concentrations of neat fragrance in the final soap bar that could cause sensitivities or allergic reactions in the people who use it.
Try the Hot Process method of soap making. This involves cooking the soap to the point where the caustic soda has all been neutralised at which point the fragrance usually has very little effect on the mixture. The downside of this is that often the cooked soap is of such a thick consistency that it is very difficult to ensure an even mixing of the fragrance. And of course the cost of the extra energy required. Hot process is as much an art as cold process - more so in many ways, so experimentation may well be required. With Hot process soap you may not need to use quite so much fragrance either... but again this depends upon how cool you allow the soap to become before adding the fragrance.
It is not a good idea to use more than 3% of the total base oils of your recipe as fragrance. Whether essential oil or Synthetic. However I do not like to give hard and fast rules about quantities of scent. You need to trust your nose, educate your nose, how? Simply smell lots of things... over time your nose gets better at it! But I digress.
On this website I have suggested 2% as a good middle ground for the amount of fragrance to use, but if you have a very strong smelling perfume then you will find that you may not need to use quite so much. Since fragrances are not cheap and the real skincare value of your soap is in the base oils that you use it makes sense to use only the minimum amount of fragrance.