Not left on the withering floor as long as other malt and spread in the drying kiln no more than 1.5 inches (37.5 mm) thick. Initially the heat was moderate, but when all the moisture in the malt was gone, the heat was suddenly increased by adding oak or beech wood to the fire. The sudden heat caused the grains to swell by 25%. The smoke from the wood gave the finished malt a smoky flavour.
The deliberate addition of wood to create smoke and allowing it to come into contact with the malt is very different from 18th century practice, where every attempt was made to prevent this happening. Though with the much-reduced proportion of brown malt being used in Porter and Stout - a maximum of 20% - the smoky effect would have been much less than in a beer made from 100% brown malt.
The method of making brown malt was changing, for a variety of reasons, one of which was the high risk of a fire.
"it was formerly the custom to dry brown malt also on ordinary kilns, with wire floors, but the labour on these was of a most disagreeable and exhausting character, and brown malt is now generally dried in wire cylinders."
The presence of diastase in older forms of brown malt is explained by the way it was produced. Diastase is much more sensitive to heat when moist. By first removing all the moisture from the malt at a low temperature, the diastase was not damaged as much by the finishing high heat.
Other coloured malts were produced in a very different way. To get the desired aroma in the malt, it needed to be heated to 160º F while it still had a moisture content of between 12 and 15%. If the moisture content was below 7 or 8%, the aromas would not be formed and all.
Though London brewers remained loyal to the malt behind the 19th-century Porter revolution, it was rarely present in Stouts brewed outside the capital. I have seen examples of its use in other styles, such as Mild Ale and Burton Ale, but these are relatively rare.