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- " As soon as he [Doc] had stopped the locomotive, he instructed Marty to climb down and hand him three cylindrical, cloth-covered packages that Doc had stashed in the DeLorean the day before. Marty picked up the first of the three, wrapped in green cloth with a big number 1 printed on it. It was about a foot and a half long and nine inches around, and was surprisingly heavy for so small a package. / Marty passed the green package up to Doc, then retrieved the second, yellow-covered cylinder. He gave that to Doc in turn, then picked up the last of the three, wrapped in red. / 'What are these things anyway?' Marty asked as he passed this one, too, to Doc. / 'My own version of Presto Logs,' the inventor replied as he stacked the red cylinder next to the other two in a corner of the cab. 'Compressed wood with anthracite dust, chemically treated to burn hotter and longer. I use 'em in my forge so I don't have to stoke it.' He pointed at the large number 3 on the red log. 'These three in the furnace will ignite sequentially, make the fire burn hotter, kick up the boiler pressure and make the train go faster.' "
- —From Back to the Future Part III by Craig Shaw Gardner (quote, page 184)
A Presto log (originally Pres-to-Log) was an artificial fuel for wood-burning stoves developed in the 1930s as a means of recycling the sawdust from sawmills. They were made of clean, dry sawdust, wood shavings and green waste that was formed into logs by machines under great pressure without any binders or glues. Other brands of artificial logs used paraffin or natural binders.
Emmett Brown developed his own Presto logs sometime in 1885 from pressed wood and pulverized anthracite (a hard and dense variety of coal), which he treated with chemicals known only to himself. The logs were color-coded green, yellow and red in increasing order of the amount of heat each produced, and bore the numbers 1, 2 and 3 respectively. Doc used them secretly in the furnace of his blacksmith's workshop to avoid having to continually feed wood to the fire.
These logs were used to power the steam locomotive that pushed the DeLorean time machine up to 88 m.p.h. Each colored log would react differently, getting the fire hotter, thus raising the steam pressure in the boiler, making the locomotive run faster than it ever could on conventional fuels. With each colored log, the locomotive would emit smoke of that color that mingled colorfully with the steam expelled from the smokestack.
The green log ignited at 500° Fahrenheit and pushed the locomotive up to 35 m.p.h. and a firebox temperature of around 1400°. At this point the yellow log fired and increased the speed all the way to 70 mph, as well as causing the temperature to soar to 2000°. At this point the red log explosively ignited, causing superficial damage to the locomotive's smokestack and headlamp. As the temperature escalated to dangerous levels, the extreme heat blew off the firebox door and produced such a surplus of steam that the boiler began to lose its structural integrity, popping off rivets and threatening to explode. The speed increased to 88 m.p.h. just before the locomotive reached the end of the track, causing the DeLorean to enter temporal displacement. This left the locomotive in a wreck of a condition, and it plunged off the end of the incomplete bridge over Shonash Ravine. Just before impact with the ground, the boiler seemed to finally self-destruct. This is quite possible, as the locomotive's fall would have caused all the water in the boiler to surge forward, exposing its firebox crown, which without the cooling effect of the water would have rapidly exceeded its tolerances and imploded, triggering a catastrophic boiler explosion. This suggests that the temperature did indeed briefly reach 2500° Fahrenheit, the point at which the steel/iron firebox of the locomotive would have melted.