Cask Consolidation, Angel's Share Calculators, and Brand Claims

Mar 21, 2023

I recently received an email from Nikkhil of the Whisky Flu blog, asking me about Camikara, a cane juice rum from India aged for 12 years. Among the marketing copy is that only 6.6% of the rum remained after the angels took their share. That seemed high to me, but not enough to immediately drop everything and investigate.

Shortly after, I received another angel’s share-related email inquiring about the 1994 Appleton’s Hearts collection. Specifically: “How is it that 12 barrels of tropical aged rum, aged for 26 years, still hold 3,000 bottles?

Both are excellent questions, easily investigated with some simple math, and an impetus for me to polish up an old Excel spreadsheet which I later turned into two online angel’s share calculators.

But before introducing these tools, let’s first clear up a common misunderstanding about rum aging. After briefly reviewing the angel’s share, we’ll look at cask consolidation, an important part of the aging process unfamiliar to many. Then we’ll look at the calculators and address the above two questions using data from the calculators.

The Angel’s Share – A Brief Review

While casks are conceptually watertight, their walls are ever so slightly porous, slowly allowing outside air into the cask. If air can enter a cask, rum can get out. Colloquially, this evaporation is called the angel’s share.

The rate that rum evaporates depends heavily on ambient conditions, particularly the temperature and its variation over time. In colder climates like that of northern Europe, casks lose about two percent of their contents during the first year, dropping to one to 1.5 percent annually thereafter. In hotter tropical climates like the Caribbean, casks lose between seven and ten percent annually.

Naturally, the volume of rum remaining in a cask after ten years is measurably less than what went in. This significant difference in evaporation rates between Europe and the Caribbean leads some to say that rum in the Caribbean ages three to five times faster than rum in Europe, e.g., five years in the Caribbean is equal to fifteen years in Scotland.

To further refine our understanding of the angel’s share, it’s important to understand that the evaporation rate slows over time. Consider a hypothetical cask in the Caribbean. In the first year or two, the lighter, more volatile compounds that can escape through a cask’s wall are the first to evaporate. In subsequent years, the heavier, less volatile compounds cannot escape as easily. Thus, the angel’s share rate decreases over the first few years before it plateaus, typically around six percent annually.

Finally, it may seem obvious to state, but the angel’s share evaporation can only act on what is currently in the cask. Say a full cask of 100 liters loses 6 liters the first year with a 6% angel’s share rate. The next year, the 6% evaporation only applies to the 94 liters that remain, so only 5.6 liters evaporate. Think of it as compound interest, but in reverse!

Cask Consolidation

Every so often, I’m asked some form of this question: If a rum aged for 30 years, there would be almost nothing left in the barrel. So how can a brand bottle hundreds of bottles of this rum? The simple truth is that this scenario rarely happens exactly like this.

When rum goes into a cask, the cask is usually filled up to the very top, with very little “empty” space remaining. But as the spirit evaporates, the empty space, aka headspace increases.

The more headspace a cask has, the higher the annual angel’s share rate. Not immensely higher, but enough to make a difference when you’re sitting on millions of liters of casked rum evaporating away. Keeping casks full minimizes evaporative losses and increases profits.

Toward that end, some producers occasionally consolidate casks of the same age. This entails emptying a set of partially filled casks into a tank, then adding that rum back into fewer casks. For example, ten casks that are 70 percent full can be consolidated into seven full casks.

Because many countries only allow age statements to refer to the youngest drop of rum in a bottle, rum makers typically only consolidate casks holding rum of the same age, so as not to diminish the declared minimum age.

Generally speaking, producers specializing in longer-aged rums will undertake cask consolidation. If a rum ages for five years or less, a producer may not bother.

The Angel’s Share Calculators

To help fellow enthusiasts with angel’s share math, I created two online calculators:

The first calculator determines how much rum should be left after a given amount of aging.

The second does the opposite: Starting from an ending quantity of rum, it shows how much rum had to begin aging ‘X’ years ago to produce the final quantity.

Unfortunately, Substack doesn’t have the ability to let me embed them here, but the links above or the images below will take you to them.

The red cells on the left side of each calculator are for users to input their values. The results columns to the right then update.

Since the angel’s share may decrease after a few years, the first calculator lets you specify both an initial rate and a rate for subsequent years.

Regarding the volume field, the units don’t matter. You can use liters, gallons, casks, or whatever. The results are in those same units.

I strongly encourage using these calculators on an actual computer rather than a mobile device. They work on a phone, but editing spreadsheet cells on a phone can be frustrating. (You have been warned.)

Both calculators are extremely simple, with few frills or safety rails. If you accidentally edit a wrong cell, simply reload the page.

The calculator engine is hosted in Microsoft’s OneDrive. If the page is slow to load or doesn’t load at all, it’s likely a OneDrive issue which I have no control over.

More Disclaimers: The calculation formulas don’t use exponential math, which would give slightly more accurate results. I’ve chosen reasonable rounding for most cells, but feel free to change them to fit your needs. The worksheets are downloadable via a button at the bottom, so you can extend or fix them to your heart’s content. As a former MSFTie, I used Excel/Office365 rather than Google Sheets. All calculated data worth price paid. If you find mistakes in my calculations, let me know.

Example: Camikara Rum

The initial question at the top of this post involves Camikara rum and how only 6.6% of the original rum remained after 12 years. Naturally, we’d like to know the angel’s share rate if so. A few moments of trying different values in the angel’s share loss calculator provides an answer: around 20%.

I know India can be extremely hot, but so are the southern Caribbean and equatorial South America. In talking to many rum producers there, the typical range is between 7% and 12% annually. If there is an explanation regarding how Camikara got a 20% angel’s share, I’d be very interested in learning more.

Example: Appleton Hearts 1994

The second question, 3,000 bottles of 1994 Appleton Hearts aged 26 years, is a good demonstration of the inverse angel’s share calculator. The question we want to answer: How much rum was laid down in 1994 to create today’s 3,000 bottles?

My friend Hurricane Hayward has noted that according to Joy Spence, the angel’s share loss in Jamaica is 6% annually and that Appleton undertakes cask consolidation every three years. Those values are consistent with what other producers have shared with me.

Appleton Hearts 1994 is bottled at 60% ABV, so presumably cask strength. Each of the 3,000 bottles is 0.7 liters, so 2,100 liters were bottled. The rum was presumably in 200-liter ex-bourbon casks, meaning 10.5 casks were required.

Per the calculation above, 10,492 liters of rum would have needed to be laid down 26 years prior. This works out to 52.5 casks of rum, i.e., 10,492 liters / 200 liters per cask = 52.5 casks.

The curious reader will note that there are other Appleton Hearts expressions available with sufficient data provided to use these calculators.

Conclusion

Ronald Reagan was known to say, “Trust, but verify.” It’s great that many producers now provide technical details and insights into their rum making. And as dedicated enthusiasts, we should ask, “Do the details they provide make sense?”

In explaining cask consolidation and creating two angel’s share calculators, I aim to add to enthusiasts’ toolsets to help them further assess the veracity of what brands say.

Note: The two above sections on the angel’s share and cask consolidation are adapted from Chapter 8 (“Aging”) of Modern Caribbean Rum.