Spile
A peg controlling gas flow in cask ale and tree tapping.
A spile, also known as a cask peg, is a peg made of wood or metal. Its primary job is to regulate the flow of air into, and carbon dioxide out of, a cask of ale or wine. Spiles are also used to tap liquids from trees, such as maple sap.
In cask ale service, the beer is dispensed without added gas. The yeast in the cask produces enough dissolved carbon dioxide to create bubbles, and can actually generate more gas than needed. To manage this, the cask is vented through a hole in the shive. When a cask is first connected to a beer engine, it is sealed. It has a keystone for the tap and a shive where it was filled. Once positioned for serving, a venting punch pierces the shive, breaking the seal, and the tap is driven into the keystone.
To let excess gas escape, a soft spile is used—made from open-grained softwood, bamboo, or harder wood with cuts. Once the cellarman decides the gas level is right, a hard spile (made from denser, more solid wood) is inserted to stop gas from escaping. When beer is drawn, the hard spile is removed and may be replaced with a soft spile or left open. When not dispensing, a hard spile is put back. Plastic spiles are also available.
After a spile is reinserted, the yeast produces enough gas to replace what came out of solution, but this production is limited. Therefore, a hard spile is kept in as much as possible. This is why real ale has a short shelf life once a cask is opened—if too much carbon dioxide is lost, the beer goes flat. Typically, the beer is good for only two to four days, so a pub must have enough turnover to empty casks while they are still at their best.
The word "spile" also has other meanings: a wooden stake or fence post; a tapper used to tap trees for sap, syrup, rubber, or palm wine; an iron or wood spigot driven into a sugar maple for syrup; and, in Indian Harbour, Nova Scotia, the shores where fish-processing "stores" were located, called "spiles."
In popular culture, a spile is used to obtain water in the novel *Catching Fire* (2009) and its film adaptation *The Hunger Games: Catching Fire* (2013). This spile is a tube with one sharpened end and the other split, meant to be pounded into a tree for a stream of fresh water.
- Shelf life after opening
- 2 to 4 days
- Materials
- wood (softwood, hardwood, bamboo), metal, plastic
- Types
- soft spile, hard spile
- Uses
- cask ale, tree tapping (maple sap, birch sap, rubber, palm wine)
Lore & Background
In cask ale use, the cask is sealed when first connected to a beer engine, with a keystone (for the beer tap) and a shive (through which it was filled). Once laid down for dispensing, a venting punch pierces the shive, breaking the seal. A soft spile (made of open-grained softwood, bamboo, or harder wood with cuts) is used to allow excess carbon dioxide to escape. When the cellarman judges this process complete, a hard spile (denser, more solid wood) is inserted to stop gas loss. When beer is drawn, the hard spile is removed and may be replaced by a soft spile or left open; when not dispensing, a hard spile is put back. Plastic spiles are also available.
Once a spile is reinserted, enough gas is generated by yeast to replace the volume lost, but the amount of carbon dioxide producible is limited, so a hard spile is kept as much as possible. This contributes to real ale's short shelf-life once a cask is opened—typically two to four days—because if too much carbon dioxide is lost, the beer becomes flat. Stronger beers last longer due to increased alcohol content acting as a preservative, though they may be a little flat by the end.
The word "spile" also has other meanings: a wooden stake or fence post; a tapper for tree sap (birch, maple, rubber, palm wine); an iron or wood spigot for maple syrup; and in Indian Harbour, Nova Scotia, the shores where fish-processing stores were located were known as "spiles."
Reader's Guide
The spile is significant in cask ale dispensing because it allows the cellarman to manage carbonation without external gas. Properly conditioned cask ale has enough dissolved carbon dioxide from yeast to have bubbles, but yeast can produce more than required, necessitating venting through the shive. The soft spile releases excess gas, and the hard spile seals the cask to preserve carbonation. This process is critical because once a cask is opened, the beer is good only for two to four days; pubs must have sufficient turnover to empty casks while still at their best. The spile's role in tree tapping is also notable: it is used to broach liquids like maple sap, and in popular culture, a spile was depicted in the novel *Catching Fire* (2009) and its film adaptation (2013) as a tube pounded into a tree to obtain water, though this technique only works in early spring or late winter when sap runs high. The taps placed in maple trees are inserted into drilled holes, and the resultant fluid is sap, not water.
Did You Know?
- A soft spile is made of open-grained softwood, bamboo, or harder wood with cuts, while a hard spile is denser and more solid.
- Once a cask is opened, real ale typically remains good for only two to four days.
- The word 'spile' also refers to a wooden stake, fence post, or the shores of fish-processing stores in Indian Harbour, Nova Scotia.
- In the novel *Catching Fire* (2009), a spile is used to obtain water from a tree, though this only works in early spring or late winter.
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