The first major step in brewing beer is to extract the fermentable sugars and starches from the malt kernels. This process is called mashing (from the German maisch: to mix). The basic idea is to harness the enzymes already present in the grain in order to digest the complex inert starches into fermentable simple sugars. If you don’t do this, the yeast will have nothing to feed upon and your brew will not ferment. We do this by mixing our grain with warm (not hot) water and allowing the reaction to proceed naturally.
Bear in mind that in the 18th Century, none of this chemistry is understood. Brewers practiced their art from techniques passed down from master to apprentice and the work is judged by how it looks, feels, smells, and tastes. It takes lots of practice for the apprentice to learn to subjectively and qualitatively judge their wort. Today, of course, we use analytical chemistry.
The brewer must be careful not to allow the mash to get too hot as this destroys the enzymes needed for conversion. Similarly, the mash cannot be allowed to get too cool or else the reaction will slow or stop. So, without the benefit of a thermometer, the Colonial Brewer must raise the temperature of his or her wort to 150°F and keep it there for about an hour. Again this is an art, passed from master to apprentice and largely judged with the five senses (see Brewing in the 18th Century Manner: Managing Heat and 18th Century Brewing: Keeping Track of Time).
Modern malt has been genetically engineered to have more than enough enzymatic activity to fully convert the available starches on other complex carbohydrates in the malt into fermentable sugars using the basic mashing process but that was not always the case. The Colonial Brewer of 1770 was forced to help the mashing process along if they wanted to get good yields from their brewing. This was done by selectively and adding heat to small aliquots of the mash then returning those aliquots to the overall mash in a process called decoction (from the Latin decoque: to boil down).
In decoction, we are using extreme heat to open up the intertwined and coiled masses of branched starches so that the natural enzymes can access and break them down into simpler sugars. We do this by pulling small samples (aliquots) of 10-25% of the total mash (water and bulk grain), heating this until it boils. This aliquot is then returned the mash, stirred in, and the process is repeated until a total volume of at least half that of the total mash has been decocted. Decoction has some benefits and some drawbacks. Decoction allows us to make beer from older or less viable grains but it also kills some of the natural enzymes and adds color and texture as the sugars in the malt become caramelized.
After the mash has had time to break down the complex carbohydrates in the malt, the brewer must then separate these fermentable sugars from the grain husks. We do this by lautering (from the German lauter: to wash). Lautering involves slowly pouring boiling water through the mash and filtering away the bulk material so that the resulting sweet wort is clear and ready to boil.
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Michael Carver -- Historical Interpreter/Reenactor 