These tunnels, or lehrs, are about 40 ft. long, and as the glasswares travel through on the trays they are subjected to the gradually diminishing heat, until they are ultimately removed at the cooler end in an annealed condition, in which state they are less liable to fracture in use. The time occupied in travelling through the lehr is usually about three days. But this period varies according to the nature of the ware being manufactured. In special glasses, and in the annealing of optical glass, the glass may undergo a process of annealing that takes as long as ten days, and in other cases, where the glassware is made very thin, no annealing at all is necessary. Usually the thicker and heavier articles require the longest time in annealing. Table glass which is made thick and heavy for cutting or decoration requires a little more care and time in the lehr than ordinary plain glassware, as the abrasive action of cutting quickly develops any latent strains and causes fracture. In some works, especially on the Continent, several small externally-heated kilns are used for annealing, in which the hot glassware, as it is made, is packed in tiers; when full, these kilns are closed up and then allowed to cool of their own accord; after which they are opened and the goods taken out to the warehouse. This is an intermittent process of annealing, and is quite satisfactory for certain classes of goods, such as lamp shades, which are usually of equal thickness throughout their form. The travelling or continuous form of lehr admits goods of more unequal thickness in form and variety. Thus, wine-glasses, jugs, and bowls may be annealed together with less risk of malformation in their shape than would be present if they were annealed together in kilns. The manufacturer can, by suitably arranging the temperature of the gas burners, give more heat to one side of the lehr than to the other. He then places the heavier goods on the hotter side and reserves the other for lighter goods, such as wines, etc. They then travel down together side by side under the most suitable conditions for the annealing of each class. Many physical changes take place in the glass passing through the lehr. One remarkable effect is the slight change in colour which occurs in glass decolorized with manganese. It is noticed that the glass becomes a greener tint in passing through the lehr when the decolorization is just on the margin of efficiency.