The Penicillin arrives looking simple. A short pour of pale gold over one large cube, in a rocks glass, with a thin darker layer resting on top. That top layer is the point. It is a small float of smoky Islay whisky, and the glass decides whether you meet it first or lose it in the first sip.
This guide answers one question: what glass a Penicillin wants, and why. It covers where the drink came from, the published build, how ice and rim shape the smoky float, the capacity that leaves room for both drink and cube, and what changes if you reach for a coupe or a tall glass instead. It sits beside our note on What Glass for a Whisky Sour, since the Penicillin belongs to the same family of shaken whisky, citrus, and sweetener.
The short answer
Serve a Penicillin in a rocks glass, also called an old fashioned glass, over one large cube or a few large chunks of clear ice. Wikipedia's entry lists the old fashioned glass as standard drinkware and the drink as served on the rocks. Difford's Guide calls for a pre-chilled old-fashioned glass and a large cube or chunk of block ice. Serious Eats, Saveur, and Epicurious all strain it into a rocks glass.
Choose a bowl of roughly 300 to 350 ml with a wide rim and a heavy base. That holds the drink, the cube, and a little air above the float, and it puts your nose directly over the smoke.
Where the Penicillin comes from
The Penicillin is young for a drink that feels classic. Difford's Guide dates it to 2005, created by Sam Ross at Milk & Honey in New York. Epicurious notes that Ross had moved to the United States from Melbourne the year before. By 2014, the same piece says, the drink was so well known that some younger bartenders assumed it was an old classic.
Ross has told the origin more than once. In Men's Journal, he describes a batch of new Compass Box releases arriving at the bar, using the mild blend Asyla as the base of a riff on the Gold Rush, and splitting the sweetening between honey and fresh ginger syrup. Then he reached for the smoky Peat Monster and floated a bar spoon of it on top of the large ice.
The Washington Post records the reason that float worked, in Ross's words: "with the gigantic rocks of ice that we would serve our drinks over, that little pool of smoke and peat sat on top of the drink and hit your nostrils before you tasted the mild, balanced concoction underneath." The glass and the ice were part of the idea from the first night.
The published build
Sam Ross's recipe, as Difford's Guide prints it, is 60 ml blended Scotch whisky, 22.5 ml fresh lemon juice, and 22.5 ml honey and ginger syrup, shaken with ice and strained into an ice-filled old-fashioned glass. A 7.5 ml float of peated Islay single malt goes on top. Candied ginger on a skewer is the usual garnish.
Serious Eats suggests a home shortcut: muddle a few slices of fresh ginger in the shaker and use plain honey syrup, since the bite of fresh ginger fades quickly in a stored syrup. If you muddle, Serious Eats also suggests double straining through a fine tea strainer so flecks of ginger stay out of the glass.
Add it up before you choose a vessel. The spec is a little over 110 ml of liquid before the shake, and shaking with ice adds water. Then the cube goes in. A glass that looks generous on the shelf can run short once all three are in it.
Why the rocks glass wins
A rocks glass is short, wide at the rim, and heavy at the foot. Each of those traits serves the Penicillin.
The wide rim puts your nose over the float
The float sits on the surface. A wide opening exposes more of that surface and brings it close to the nose on every sip. Epicurious quotes Ross on this directly: he preferred never to serve the drink with a straw, because he wanted the smoke in the nose and the spicy, sweet cocktail underneath. A rocks glass does that work without asking anything of you.
The short wall keeps the drink and float together
In a tall glass, the same drink sits low, far below the rim, and the smoke has further to travel before it reaches you. In a short bowl, the surface sits closer to the lip. You smell the Islay and then taste the honey, lemon, and ginger, in the order Ross described.
The heavy base steadies a drink you carry
A float is a layer, and layers dislike sloshing. A thick foot lowers the centre of gravity, so the glass sits steady on the table and travels from bar cart to chair without stirring the smoke into the rest of the drink.
Large ice and the float
The large cube does two jobs. It chills a shaken drink without diluting it quickly, and it gives the float a still surface to rest against. Ross's own account and the Difford's spec both point to big ice. A Club Oenologique feature on the drink reports that Milk & Honey always poured it over a large, clear-cut cube.
The same feature shows that bars now vary the ice. Some use standard small cubes. One serves it over large chunks broken from a clear block, which, the bartender explains, chills as well as one big cube while managing dilution more slowly than small cubes. Any of these works in a rocks glass. What changes is pace: a single cube or a few big chunks keep the drink tasting like itself for longer.
If you make ice at home, our notes on How to Make Clear Ice at Home for Whisky and Sphere Ice vs Cube cover clarity and shape. For the Penicillin, a cube is the easier choice. Its flat top gives the float a calm place to settle, and Serious Eats pours the float over the back of a bar spoon so it lands gently.
Capacity: leave room for the cube
Rocks glasses vary widely in size. For a Penicillin, a small one is tight. The shaken drink, the cube, and a finger of headspace together want more room than a small tumbler offers. One recipe in the Club Oenologique piece names a 12 oz rocks glass, which is about 355 ml.
At home, aim for 300 ml or a little more. Fill the glass with water first, add your usual cube, and see where the level sits. If it reaches the rim, the glass is a neat pour glass, not a Penicillin glass. Our guide to What Size Whisky Glass Do You Need walks through the same test, and What Is Headspace in a Whisky Glass explains why the air above the drink matters for aroma.
Difford's Guide calls for a pre-chilled glass, and a short dry rest in the refrigerator is an easy way to get there. A cold bowl keeps the cube from melting quickly in its first minutes. Our note on Should You Chill Whisky Glasses covers how.
What about a coupe or a tall glass
You can serve a Penicillin up, in a coupe, without ice. It still tastes like a Penicillin, but it is a variation, not the published spec. Without ice, the drink warms faster, and a shallow stemmed bowl with a float on top is easy to spill. Drink it promptly if you choose this route.
A highball or tall tumbler is the wrong shape for this drink. There is too much glass for a short cocktail, the surface sits deep, and the smoke reaches you late. Save the tall glass for the long serves in our Mizuwari vs Highball guide.
A tasting glass, such as a Glencairn, suits a neat Islay malt rather than this drink. The narrow mouth concentrates aroma, but there is no room for ice. If you want the smoky Islay on its own, pour it neat. If you want the Penicillin, use the rocks glass.
What cut glass does with a Penicillin
The Penicillin is pale gold, close to the color of the honey in it. In a plain glass that color reads as one flat field. In a cut rocks glass, the facets break it into small planes of light, and the large clear cube sits inside the pattern like a window.
The float shows best when the cut stops below the rim. A smooth band at the top lets you see the darker layer resting on the surface. A smoke overlay frames the drink in grey and echoes the peat. An amber band or base repeats the honey tone. The object is still. The light is doing the moving.
When you finish, tip out the ice and rinse the glass promptly. Honey and lemon are sticky when they dry in the cuts. How to Care for Cut Glass has the full routine.
A few common questions
What glass is a Penicillin served in?
A rocks glass, also called an old fashioned glass, over one large cube or large chunks of ice. That is the glass named in Sam Ross's recipe as Difford's Guide prints it and the standard drinkware listed on Wikipedia.
What size rocks glass do I need?
About 300 to 350 ml is comfortable. It holds the shaken drink, a large cube, and a little headspace above the float.
Can I use small ice cubes?
Yes. Some bars do. The drink will dilute faster, so drink it a little sooner. A single large cube or a few big chunks keep the balance longer.
Why not use a straw?
Ross himself preferred no straw, according to Epicurious. A straw pulls from the bottom of the glass and skips the smoky float on top, which is the first thing the drink is meant to give you.
Is a Penicillin a good autumn drink?
Serious Eats calls it a cure for a chilly autumn night, and the honey, ginger, and smoke do suit cooler evenings. It is served cold, over ice, in the same rocks glass all year.
Rocks glasses with room for a large cube
- Smoke Orb Whisky Glass: 320 ml, smoke walls with a checkerboard of asa-no-ha, or hemp-leaf star lattice, and faceted dot fields over a thick clear base.
- Smoke Wave Whisky Glass: 350 ml, a clear upper bowl cut in rokkaku-kagome, or hexagonal basket weave, a polished amber wave, and a smoke base, so honey and peat are both in the glass.
- Smoke Lightning Whisky Glass: 330 ml, smoke walls with an asa-no-ha band, generous room for a large cube and a float.
- Smoke Deep Whisky Glass: 300 ml, yarai, or bamboo-fence lattice, from rim to foot, with a smooth band of color at the rim where the float rests.
Each of these listings is Edo Kiriko: handmade crystal, hand-cut in Tokyo, as its product page states. View all glassware in Whisky Glasses.
Related reading: What Glass for a Whisky Sour · Large Ice and the Rocks Glass · Old Fashioned vs Rocks Glass