The word vitriol comes from Medieval Latin vitriolum, derived from Late Latin vitreolus ("glassy") and ultimately from Latin vitrum ("glass"), and it entered Middle English via the Old French vitriol.  Early alchemists observed that metallic sulfates (like blue vitriol for copper sulfate or green vitriol for iron sulfate) formed shiny, glass-like crystals, and this term was extended to concentrated sulfuric acid, known as "oil of vitriol," due to its corrosive, glass-eating properties.  Because sulfuric acid eats away at things, the word "vitriolic" came to mean harshly critical or full of bitterness, much like the acid burns.  This led to the figurative meaning of vitriol as bitter, scathing speech or notes. 

According to the Oxford English Dictionary (OED), the noun vitriol first appeared in English during the Middle English period (1150-1500j),  with the earliest recorded use found in the writings of Geoffrey Chaucer around 1386.  It originally only referred to substances like "green vitriol" (iron sulfate) or "blue vitriol" (copper sulfate), used in alchemy and medicine.  It later expanded to include the sense of "bitter, caustic, or harsh" feelings/speech, attested first in 1769, referencing the corrosive nature of the chemical.  The adjective "vitriolic" (harshly condemnatory) emerged in the late 17th century (around 1670). 

Examples of "vitriol" used in a sentence: 
"It was almost as if the judge's words were spraying vitriol on her face." 
"It starts out as debate and ends up full of hate, name-calling and vitriol." 
"He was shocked by the vitriol in Washington and by his own poor judgment." 
"His lengthy speech was full of political vitriol." 
"Julie Gilbert recalls some of the vitriol in her new book." 
"They have both had a big fat load of smug vitriol heaped upon their heads." 
"They think they can replace their own responsibility for the failures they delivered with personal bile and vitriol directed at me."