Seven oils, a 50 ml batch, a scale and a spreadsheet. The final lesson of the blending course converts a formula from drops to grams and percent, checks each material against IFRA and dermal limits, then costs the very bottle made in lesson seven.
The drop is an uneven unit
The previous seven lessons counted in drops, because the drop is the easiest unit when working on blotters. The library works on one drop being about 0.05 ml, or 20 drops per ml. The same convention carries a margin of error: about 20 percent, depending on viscosity, temperature and dropper type.
A 20 percent margin hardly matters when the aim is to compare two blotters. It matters a great deal when a formula has to be repeated exactly, or when a material sits close to a safety limit. Thick oils such as vetiver form larger drops; thin citrus oils form smaller ones. Two formulas that both say ten drops can hold two different amounts.
The library's formula calculator says it plainly: for small amounts, weighing in grams is more accurate than counting drops. Professional formulas are usually weighed in grams and written in parts per thousand or percent of the jus. Drops remain useful for quick tests but should not be the unit a formula is kept in.
Lesson eight therefore adds no new oil. It takes the formula that has travelled through seven lessons, from the ten bottles of lesson one to the bottle rested in lesson seven, and rewrites it in units that do not change from bottle to bottle.
Converting to grams and percent
The first step is to write the jus as percentages totalling 100. The example formula for this lesson: sweet orange 30, coriander seed 5, clary sage 10, damask rose 3, geranium 17, patchouli 30, vetiver 5. Sweet orange sits in the top, rose and geranium in the heart, patchouli and vetiver in the base. Coriander seed and clary sage are the two modifiers tested in lesson five.
Geranium here carries most of the floral tier. The library's substitution chart notes that geranium shares many molecules with rose, such as citronellol and geraniol, and is often used to hold the floral tier when rose has to be reduced. Why rose is down to 3 percent becomes clear in the limit check.
The second step is to choose the batch and concentration. A 50 ml batch at 15 percent holds 7.5 ml of jus and 42.5 ml of alcohol. Each line of the jus multiplied by 7.5 ml gives its volume: sweet orange 2.25 ml, rose 0.225 ml, and so on.
The third step converts volume to grams through density. The formula calculator defaults to 0.9 g/ml and notes that essential oil densities commonly fall between 0.85 and 1.05. At that default, 0.225 ml of rose is about 0.2 grams. When the real density of each lot is known it should be entered per line; some materials, such as cinnamon, clove and benzoin, are heavier than water.
Checking each material against its limit
Once the formula is written in percent, checking limits is only multiplication. The share in the jus times the concentration gives the share in the finished product. Rose at 3 percent of the jus, at 15 percent, is 0.45 percent of the product. Geranium at 17 percent of the jus is 2.55 percent. Patchouli at 30 percent of the jus is 4.5 percent.
Each figure is compared with the limit on that oil's page. Damask rose has a maximum dermal level of 0.6 percent; 0.45 sits below it. Geranium has 17.5 percent; 2.55 is far below. Patchouli has no IFRA concentration limit of its own. The formula calculator does not do this step; the Safety page and each oil page are where the limits are looked up.
The second check goes down to molecules, since several oils carry the same compound. Geraniol has an IFRA limit of 5.3 percent in leave-on products. In this formula geraniol comes from geranium, at most 30.3 percent; from rose, at most 25.7 percent; and small amounts from coriander seed and clary sage. Adding up the maximum levels, geraniol in the finished product stays under 1 percent.
The real limiting compound is methyleugenol. Damask rose contains up to 3.3 percent methyleugenol, and the IFRA limit for fine fragrance is 0.02 percent of the finished product. At 0.45 percent rose, methyleugenol comes to about 0.015 percent. That is why rose stops at 3 percent of the jus. If the same jus went into a body lotion, the methyleugenol limit would be 0.0004 percent and rose would almost have to leave the formula. The piece on IFRA explains why limits change with the product category.
Cost: the most expensive material decides
The formula calculator has a cost column. Each line takes the price of 10 ml of the material; the line cost is the number of ml divided by 10, times the price. The currency is whatever is entered. The right price is the price of the actual lot on the table, not a general figure.
In the example formula, the lines do not contribute equally. Sweet orange takes 30 percent of the jus, but the library notes it is the most produced essential oil in the world by volume, largely a by product of the orange juice industry, and seldom adulterated because of its low price. The largest line by volume can be the smallest by cost.
Rose sits at the other end. The library records 3,000 to 5,000 kg of flowers for one kg of oil, and premium rose otto can reach 8,000 to 15,000 USD per kg. With 0.2 grams of rose in a 50 ml batch, this line alone comes to about 1.6 to 3 USD at that price range. Three percent of the jus can be most of the cost.
The cost of a bottle is therefore usually decided by one or two of the most expensive materials, not by the number of bottles on the table. When cost has to come down without changing the direction of the scent, the first line to review is the most expensive one, and the substitution chart is where to begin. Geranium stands in this formula partly for that reason.
Scaling a formula to a batch
A formula written in percent has no size. It can become 10 ml, 50 ml or 500 ml without changing. Blending 101 gives beginners a simple scaling method: ten drops become one hundred, that is 5 ml. Once written in percent, scaling is just changing the figure in the finished batch field.
Scaling also exposes errors that a small batch hides. In the 50 ml batch, rose is 0.225 ml, about 4.5 drops. With the 20 percent margin of a drop, the real amount could run from about 3.6 to 5.4 drops, putting rose in the finished product anywhere from about 0.36 to 0.54 percent. This formula stays under the limit at both ends, but a formula closer to the limit would not.
A real batch is therefore weighed, not counted. The weighing usually runs from small amounts to large: lines under one gram are weighed first, while errors are still easy to correct. Each line records both the planned grams and the grams actually weighed. The difference between the two columns is information for next time.
A large batch also needs resting and filtering as in lesson seven. Resting time does not scale with volume; a 500 ml bottle does not need ten times the time of a 50 ml one. But the large batch should be dipped on a blotter and compared with the approved small batch before it is bottled.
A complete formula sheet
The final formula sheet gathers everything from eight lessons onto one page. The top records the name, date, product type, applicable IFRA category, batch and concentration. The middle is a table: oil name, lot, percent of jus, percent of product, planned grams, weighed grams, the limit looked up, line cost.
The bottom records what the table does not hold. The limiting compound of the formula, here methyleugenol from rose. The allergens to declare if the product is sold. Resting time, filtering date, and blotter descriptions at five minutes, one hour and one day, the rhythm kept since lesson two.
The library's pyramid builder and formula calculator both store choices in the browser. The formula sheet on paper or in the notebook remains the master copy. A browser can be cleared; a dated notebook page cannot.
The eight lessons follow one order: nose, time, ratio, accord, modifier, fixative, bottle, numbers. Each lesson adds a column to the scent notebook. By the last one, the notebook holds enough for someone else to open it and make the same bottle again.
The nose decides the scent. The scale decides the formula.
One figure written correctly keeps a bottle for years.
The course ends on a sheet of paper, not in a bottle.