Ingredients for a cocktail at Friday Saturday Sunday

The fastest way to dispel the notion that art and math don’t mix is to toss back a drink by Friday Saturday Sunday head bartender Paul MacDonald.

It’s not the booze going to your head that’ll fuzz the lines between disciplines. It’s the 29-year-old Bethlehem native, who is without a doubt a visual artist — see his impressively gorgeous Instagram account, with 13.8k followers — and has come up with a novel way to create new cocktails using math. Specifically, the Fibonacci sequence.

The series of numbers, which mathematicians discovered nearly a millennium ago, shows up often in nature — seeds on a sunflower follow its pattern, for example, as do scales on a pineapple — and also in art. Ratios created by its visually pleasing spiral have been spotted in everything from the Mona Lisa to the Last Supper.

But this might be the first time the Fibonacci sequence has made its mark behind the bar.

Relying on trusted mathematical ratios to help ensure balance in cocktails isn’t a new thing. Drink makers have fallen back on standard proportions for decades. When looking for fresh combinations that please the palate, MacDonald explained, bartenders will often swap out one or more elements while still mixing the ingredients in the same proportions.

The most classic formula is approximately 2:1:1 — two parts liquor, one part sour and one part sweet — and is used in everything from a whiskey sour to a mojito. Other common ratios are the 2:1 of a Martinez or the 1:1:1:1 four equal parts that go into a Last Word.

“There are five or six different specs that make up the cocktail canon,” MacDonald said.

And now there’s another.

MacDonald has a deft hand Credit: Danya Henninger / Billy Penn

For his Fibonacci cocktails, MacDonald uses five ingredients combined in the ratio of the first five numbers in the famous sequence — 1:1:2:3:5. In keeping with the mathematical series, each number is the sum of the two prior to it. Specifically, MacDonald uses ¼ oz of ingredient A, ¼ oz of B, ½ oz of C, ¾ oz of D and 1¼ oz of E. The measurements add up to 3 oz, which is a standard cocktail pour.

“Prior to [Paul], I’d never heard of a bartender leaning on a integer sequence to build drinks,” offered local booze expert Drew Lazor, who is a regular contributor to Punch Drink and has a book called Session Cocktails coming out May 2018. “But the association makes sense as good cocktails are all about balance, and the Fibonacci numbers provide a visualization of an end product where each ingredient builds logically upon the elements that precede it.

“This progression occurs almost magically in nature, so why wouldn’t it work in a cocktail context, as well?”

Magic, indeed. Like 13th century Italian mathematician Leonardo of Pisa, who is credited introducing the series to European society and who became known as “Fibonacci,” MacDonald stumbled on the sequence by accident.

Entirely self-taught, he got into bartending on a whim, when he was looking around for a job after college and was hired at Bethlehem’s Bookstore Speakeasy in 2010. He picked up knowledge on the job, and was adept enough to score a position at now-closed Farmers Cabinet in Philly. He moved on to Society Hill Society (also now gone), and then to in Rittenhouse. It was there that inspiration struck.

“We were working on drinks with five different fortified wines,” MacDonald remembered, “and I had the idea that it would be cool to make a drink in which each of those five flavors came through — where you could taste each of these things in succession.”

He’s been working on it ever since, and took the concept with him when Chad and Hanna Williams asked him to come lead the spirits program at Friday Saturday Sunday last year. In the drink called Fibonacci in Autumn, which will show up on the Fri Sat Sun menu soon, the five ingredients are, in order, Cocchi Americano, Cappelletti Aperitivo, Laird’s 100 Apple Brandy, Green Chartreuse and Amaro Nardini — and it works as promised. The disparate parts come together in a sip that changes flavor and sharpness as you swallow.

MacDonald holds his newest Fibonacci cocktail Credit: Danya Henninger / Billy Penn

MacDonald is trying to carefully avoid selling his mathematical discovery as a gimmick, but he does credit it with making him more creative.

“If a bartender like Paul can achieve results he’s happy with by tinkering with the works of an ancient thinker,” Lazor said, “then more power to him.” He added, “I just hope he doesn’t ask me to do any math in my head after I’ve already started drinking.”

Danya Henninger was first editor and then editor/director of Billy Penn at WHYY from 2019 to 2023.