Cask Anatomy and Preparation

A Few Silly Words Associated with Cask Ale

Nitwit! Blubber! Oddment! Tweak!

Cask ale is often described in British imperial units, which are slightly different from the American units to which the reader may be accustomed. An imperial gallon is roughly 1.2 American gallons (a fact which has its own interesting history). An imperial barrel is 36 imperial gallons, or 43.23 American gallons. This is distinctly different from an American brewer’s barrel of 31 American gallons. A variety of smaller vessels, such as the kilderkin, firkin, and pin, are used to distribute the prepared cask ale in a volume more reasonable for a cellarman to physically handle. There are also a few unreasonable vessels such as the tun, butt, and hogshead typically used by the brewery for special occasions where mass consumption is expected.

A firkin (from Middle Dutch vierdekijn, meaning “fourth”) is a quarter of an imperial barrel, or 9 imperial gallons. While this does convert to 10.8 American gallons of capacity, there is usually closer to 10.6 gallons of primed beer actually contained in a filled firkin.

A pin is equal to half a firkin (4.5 imperial gallons or 20 liters). Similarly, while this converts to 5.4 American gallons of capacity there is usually closer to 5.3 gallons of primed beer. This is just a bit bigger than the Cornelius-style kegs popular in homebrewing. In this picture, the pin is on the left while the firkin is on the right.

pin and firkin

Pins, firkins, and all vessels that transport beer are collectively called cooperage. All of these words will be used interchangeably except for when a size is specified.

A stillage is where the beer is placed while it is rotated into service. For a festival, this could be the table or stand that the beer is placed upon. A commercial cellar will doubtlessly have a permanent stillage. There are a great number of ways to position the cask for service, such as chocking the cask, placing it on a stand, using an auto-tilt stillage, or using a more permanent solution. Generally a beer is placed onto the stillage, then vented, tapped, and served.

Here are two examples of stillages. On the left is a custom-built wooden pin cradle and enclosure used at Atwood’s in Cambridge, MA. On the right is a temporary stillage built with scaffolding in use at the 2013 NERAX festival.

Pin Cradle at Atwood's in Cambridge, MA

The NERAX stillage

Anatomy of a Cask

[Combine general cask terms from this image with vessel-specific terms here and here. This section is just a diagram.]

Tools of the Trade for Sealing a Cask

Woody Bits:

Plastic Keystone

Fills keyhole to seal vessel. Holds tap in place. Cheap. Works well enough with stainless. Do not use with plastic cooperage!

Wooden Keystone

Fills keyhole to seal vessel. Holds tap in place. Has the advantage of soaking up beer and expanding to seal most leaks.

Plastic Shive, Single Piece

Fills bung hole to seal vessel. Is punctured to vent headspace. Holds spile (hard or soft). Cheap. Do not use with plastic cooperage!

Plastic Shive, Banded Evolution ‘C’

Fills bung hole to seal vessel. Is punctured to vent headspace. Holds spile (hard or soft). Much improved over single piece design. Part of a series produced by Rankin.

wooden shiveWooden Shive

Fills bung hole to seal vessel. Is punctured to vent headspace. Holds spile (hard or soft). Has the advantage of soaking up beer and expanding to seal most leaks.


Seals keystone after tap has been removed. The ‘B’ in TOBI.


Useful Handtools:

Deadblow Hammer

Pounds woody bits and taps into position. Deadblow reduces recoil. Much better than traditional ash mallets.

ash malletAsh Mallet

Very traditional motivational device.

large screwdriverLarge Regular Screwdriver

Always a handy lever to have in one’s toolkit.

Shive Extractor

The easy way to remove shives.

Cleaning Your Cooperage

Casks have a tendency to get really unsanitary. In the trade, an emptied cask will be TOBI’d (tap out, bung in with a spile pounded in to seal the vessel) and left to sit unwashed for weeks if not months before rotating back to the brewery. The living yeast that worked the beer is still very much still alive. Most cask service allowed air in as the beer was drained, pretty much guaranteeing introduction of an acetobacter in addition to a potpourri of other spoilage organisms that have colonized the emptied vessel. All of these microorganisms work together to develop a unique and powerful aromatic punch. This is a completely different experience than when working with emptied kegs.

Some breweries have specialized cleaning apparatus that use some combination of scalding temperatures, strong caustics, and powerful sprayers to scour the microbial interlopers from the cask. They have to be; should any organism survive, the next beer racked in will be at great risk of being condemned before service. While this is an accepted reality in the English trade, American distributors and breweries seem to have a hard time getting the hang of the rhythm of the cleaning regimen needed for cask ale.

Homebrewers have a terrific advantage here as they maintain complete control over their cooperage. Much like with glass bottle reuse, an ounce of prevention in the form of rinsing immediately after use is worth a pound (or at least a few ounces) of cleanser. Simply rinsing away the remaining contents upon emptying the vessel dramatically reduces possible food sources for spoilage organisms. Leaving the vessel open as opposed to closed allows the interior to dry out, further frustrating these organisms. Of course, sanitizing before storage makes for an even more inhospitable environment. I wouldn’t make up a batch special, but if there’s some kicking around, it is best practice.

For the breweries that are without specific equipment or the indiligent homebrewer, cleaning a ripe cask is not particularly onerous once you get past the smell. Simply remove the old keystone and shive, then rinse with hot water (tap-hot is fine) until it runs out relatively clear. Visually check for and make note of chunks of stuck-on matter as best you can. We just want to be sure that they come off after the hot soak.

Depending on how you prefer to fill the vessel, sanitize the appropriate orifice, and insert either the keystone (for filling via the bung hole) or shive (for filling via the key hole). I prefer to fill via the key hole so that the cask stands on end without chocks. Many breweries and those adding significant dry-hops or other late flavoring agents may prefer to fill via the bung hole. It makes no difference.

Fill the vessel with hot water, the hotter the better, up to 180°F. Add a strong cleanser such as Powdered Brewery Wash (PBW) in a strong cleaning solution. Let stand and soak as directed. Drain, check that those trouble spots are now clean, then rinse with hot water to remove any residue.

Thoroughly sanitize the vessel. I prefer to use a gallon solution of StarSan because I can slosh it around and it will leave sanitizing foam clinging to interior surfaces. However, any no-rinse sanitizing solution will work. I alternate sloshing and resting in various positions to make sure that the bung area, the back end, the ullage, and the back side of the front end all get some time submerged by the StarSan solution. I have a rubber stopper (#8½) to temporarily fill the key hole while soaking the interior of the front end.

Once sanitized, drain away the sanitizer and fill immediately with fresh beer. When using StarSan, don’t fear the foam. I prefer to add my priming agents before racking the beer to ensure that they’re in there.

How to Prime a Cask

Cask ale is best served at 1.1 – 1.3 dissolved volumes of CO2 (roughly half of the usual volume of packaged beer), although it can be found served with as high as 1.75 dissolved volumes of CO2. Fermented beer has a certain volume of CO2 present as a residual from fermentation. This amount is temperature-dependent (cold beer holds on to more CO2) and can be found on the chart below.

Dissolved Volumes of CO2 Present after Fermentation

Temperature (°F/°C)Volumes CO2
47 °F (8.33 °C)1.21
50 °F (10.0 °C)1.15
53 °F (11.7 °C)1.09
56 °F (13.3 °C)1.04
59 °F (15.0 °C)0.99
62 °F (16.7 °C)0.94
65 °F (18.3 °C)0.89
68 °F (20.0 °C)0.85
71 °F (21.7 °C)0.81
74 °F (23.3 °C)0.77
77 °F (25.0 °C)0.73
80 °F (26.7 °C)0.69
83 °F (28.3 °C)0.66

We want 1.4 dissolved volumes in our finished, unbreached cask. This assures us of the 1.3 volumes we want while reducing the concern over blowing a shive or keystone. To get there, simply subtract the volumes expected to be in your beer from 1.4.

Example: Merkin’s Best Bitter has finished its fermentation at 65°F. Using the chart above, we’d expect there to be .89 dissolved volumes of CO2. Since we want to raise the beer to 1.4 volumes after conditioning, we need to create .51 (1.4 – .89) dissolved volumes of CO2 in the conditioning vessel.

The most common and easiest-to-use priming agent is dextrose. This is also known as corn sugar, dextrose monohydrate, glucose monohydrate, or glucose.h2o. It is the simplest form of glucose and the easiest one for tired yeast to digest. Dextrose, corn sugar, and priming sugar will be used interchangeably. Use the following chart to decide how much dextrose you need to create the necessary volumes of CO2:

Priming with Dextrose

DextroseVolumes of CO2
oz (g)per 5 Gallons (19L)per pinper firkin
1 (28.3)0.340.320.16
1.5 (42.5)0.510.480.24
2 (56.7)0.680.640.32
2.5 (70.9)0.850.800.40
3 (85)1.020.960.48
3.5 (99.2)
4 (113)1.361.280.64
4.5 (128)1.531.440.72
5 (142)1.71.600.80
5.5 (156)1.871.760.88
6 (170)2.041.920.96
6.5 (184)
7 (198)2.372.241.12
7.5 (213)2.542.401.20
8 (227)2.712.561.28
8.5 (241)2.882.721.36
9 (255)3.052.881.44

Example, continued: We are filling a pin with Merkin’s Best Bitter. According to the chart above, we can get the .51 dissolved volumes of CO2 by using a little more than 1.5 ounces of dextrose. (.51 is a little more than .48.) Let’s use 1.6 oz to ensure that we get where we want to go.

Simply boil this amount of dextrose in a small amount of water to sanitize and add it to the vessel as you’re filling it. A two-minute boil is sufficient. For a pin, use a cup of water. For a firkin, two. Some brewers will skip the sanitization step but this is bad practice. Your beer will never be as vulnerable to spoilage organisms as when it is being transferred to the service vessel. While certain brewpubs may be able to get away with sloppy practice by quickly turning over the beer, bad practice will never yield consistently great results.

Other fermentables can also be used as a priming agent, including unfermented wort of known specific gravity.

Alternate Fermentables for Priming

A table of various priming sugars and their efficacy.
FermentableExtract Yield (PPG)% FermentabilityConstituents
Dextrose (Corn Sugar)42100%Glucose (~5% moisture as dextrose monohydrate, 0% moisure as anhydrous dextrose)
Belgian Candi Sugar46100%Sucrose, inverted
Honey3895%Fructose, Glucose, Sucrose (~18% water)
Lactose460%Lactose (<1% moisture)
Lyle's Golden Syrup46100%Glucose, Fructose (18% water)
Maltodextrine420%Dextrines (5% moisture)
Maple Syrup31100%Sucrose, fructose, glucose (~34% water)
Molasses / Treacle36Varies, ~60%Sucrose, invert sugars, dextrines.
Rice Syrup Solids42Varies. ~80%Glucose, maltose, other (~10% moisture)
Table Sugar (Sucrose)48100%Sucrose

The selection of your priming agent can impact the flavor of the finished beer. As John Palmer put it, “Do you want to keep the priming sugar hiding in the wings or do you want to bring it onstage?” It is left to the brewer to decide if this is desirable. Using the weight of dextrose calculated above, we can determine the weight of any fermentable listed on this chart necessary to sufficiently carbonate the beer by using the following formula (also provided by John Palmer):

(Weight of A)(Percent Solids of A)(Fermentability of A) = (Weight of B)(Percent Solids of B)(Fermentability of B)

Since we’re standardizing our calculation using dextrose, the left side of the equation becomes:

(Weight of A)( 95% )( 100% ) = (Weight of B)(Percent Solids of B)(Fermentability of B)

Once an alternate priming agent has been selected, the data for Percent Solids and Fermentability come off of the chart.

Example, continued: Merkin’s Best Bitter relies on a hint of maple to give it the characteristic sweet finish. We calculate the amount of maple syrup necessary to carbonate by filling in the formula and solving.

( 1.6 oz )( 95% )( 100% ) = X ( 66% )( 100% )
1.52 oz = X (66%)
2.3 oz = X

Therefore we require 2.3 oz (65 g) of Maple Syrup to gain the .51 dissolved volumes of CO2 necessary to sufficiently carbonate the beer in our pin.

Carbonation can be obtained with pretty much any substance that contains sugars and has an FDA food label (or do a Google search for “food nutrition facts and analysis”). It’s pretty safe to assume that the dietary sugars listed on such a label will be about as fermentable as table sugar (aka cane sugar or sucrose). Calculate the necessary amount of table sugar in ounces using the conversion formula. Convert to grams (all labels give this information in grams). Calculate how many servings you need to get that amount of table sugar. Multiply the number of servings by the serving size (by weight). Convert back to ounces if necessary. Be sure to consider the impact of that much foodstuff on your beer!

Example: Wildman Wheat Beer is being put into a firkin and primed with a sour cherry syrup. It finished fermentation at 72°F. There are about .8 dissolved volumes of CO2 in solution. To get to 1.4 dissolved volumes in a firkin, we’d need 3.75 oz of corn sugar (chart 2, firkin column, .6 volumes necessary). To convert this to table sugar, we solve:

( 3.75 oz )( 95% )( 100% ) = X ( 100% )( 100% )
3.56 oz = 100.92 grams = X

The sour cherry syrup contains 25g of sugar per 100g serving size. We need just over four serving sizes (404 grams or 14.25 oz) of the sour cherry syrup to carbonate this beer.

Alternative Priming Methods

If you are intimately familiar with your beer and know exactly when it will finish fermenting, you can skip the addition of priming sugars by packaging before the beer hits final gravity (FG, the specific gravity [SG] at the end of fermentation. aka terminal gravity). This can be a bit dicey because if you package too late, your beer never carbonates. If you package too early, you could blow a shive.

Example 2: I want to package Merkin’s Best Bitter without additional sugars. What should the SG of the wort be? I know this beer well and it always finishes at 1.008. The beer is currently at 68°F so it has about .85 dissolved volumes in solution. To get to 1.4 dissolved volumes I’ll need .55 additional volumes or about 1.75 ounces of dextrose. Dextrose contributes 42 gravity Points per Pound per Gallon (PPG), so in my 5.3 gallon vessel it will contribute 7.92 points per pound per pin, which is .495 points per ounce per pin. I need 1.75 ounces to carbonate, which would add .866 gravity points to the pin. So if I package it at 1.009 sg, it should carbonate fine. To make sure, I’d rather package it at 1.010.

There is another tradition of using partially or unfermented wort to prime the beer.

Example 3: Dang! This batch hit terminal gravity before I finished my calculation! Good thing I have another batch coming along right on its heels. I still need the equivalent of 1.75 ounces of corn sugar to carbonate. If I measure the batch coming along at 1.020, this means that there are 12 gravity points remaining of fermentation. Dextrose has 42 PPG so this is the equivalent of 4.57 ounces of dextrose in a gallon of this beer. We only need 1.75 ounces of dextrose so we can get that equivalency out of .383 gallons = 49 ounces, which is about 3 pints. So I add 3 pints of the nearly fermented beer to my pin, then add the finished beer to top it up.

The yeast in solution will ferment the sugars over the next several days. Just because all of the sugars are consumed doesn’t mean that the beer is ready. I recommend allowing it to mature for another two weeks, just like when bottle conditioning homebrewed or commercial beer.

The One-Page Advice Sheet for Priming Casks

Priming Your Pin or Firkin


Harry Potter and the Sorcerer’s Stone, First American edition hardcover, 1998, p 123