The Bath School disaster, sometimes known as the Bath School massacre, was a series of violent attacks perpetrated by Andrew Kehoe on May 18, 1927 in Bath Township, Michigan, which killed 38 elementary schoolchildren and 6 adults and injured at least 58 other people.
 Kehoe killed his wife and firebombed his farm, then detonated an explosion in the Bath Consolidated School before committing suicide by detonating a final device in his truck.
 It is the deadliest mass murder to take place at a school in United States history.
Avgas (aviation gasoline, also known as aviation spirit in the UK), is an aviation fuel used in spark-ignited internal-combustion engines to propel aircraft.
 "Avgas" is distinguished from "mogas" (motor gasoline), which is the everyday gasoline used in motor vehicles and some light aircraft.
 Unlike mogas, which has been formulated since the 1970s to allow the use of platinum-content catalytic converters for pollution reduction, the most commonly used grade of avgas still contains tetraethyllead (TEL), a toxic substance used to prevent engine knocking (detonation), with ongoing experiments aimed at eventually reducing or eliminating the use of TEL in aviation gasoline.
Materia medica (English: medical material/substance ) is a Latin medical term for the body of collected knowledge about the therapeutic properties of any substance used for healing (i.e., medicines).
 The term derives from the title of a work by the Ancient Greek physician Pedanius Dioscorides in the 1st century AD, "De materia medica", 'On Medical Material' (Περὶ ὕλης ἰατρικῆς in Greek).
 The term "materia medica" was used from the period of the Roman Empire until the 20th century, but has now been generally replaced in medical education contexts by the term "pharmacology".
Plumber's putty is a type of putty used as a sealant in plumbing.
 It is a pliable substance used to make watertight seals around faucets and drains.
 The putty is a basic component of a plumber's toolkit and is often used when replacing plumbing fixtures.
 Plumber's putty formulations vary but commonly include powdered clay and linseed oil.
 Other formulas use limestone, talc, or fish oil.
 RTV silicone or epoxy sealants may be used in place of putty.
Aluminium sulfacetate is a chemical compound of aluminium with formula Al2SO4(CH3CO2)4 .
 It is a double salt of aluminium sulfate and aluminium acetate.
 It can be used as a mordant, which is a substance used to set dyes on fabrics that typically contains a polyvalent metal ion like aluminium or iron, In mixtures with basic aluminium diacetate or aluminium sulfacetate, aluminium triacetate has been used as a mordant with alizarin dye.
 In 1899, Albert Ganswindt recommended that the use of impure sulfacetates that are empyreumatic liquids "should be abandoned" in favour of pure preparations.
 Empyreuma is an obsolete chemical and medical term referring to "the smell and taste associated with burning vegetable and animal matter", and likely results in this case from the use of pyroligneous acid (wood acid) or wood acid lime in the preparation of the mordant.
Pyrotol was an explosive available for a time after World War I.
 It was reprocessed from military surplus.
 Usually used in combination with dynamite, it created an incendiary blast.
 Since it was very inexpensive, it was often used by farmers to remove tree stumps and clear ditches.
 The substance was known for being used to commit the Bath School bombing in 1927 and distribution of pyrotol for farm use was discontinued in 1928 due to exhaustion of the supply of surplus explosives.
Figure of Insensitivity (or "F of I") is an inverse scale of measure of the impact sensitivity of an explosive substance.
 In this particular context the term 'Insensitivity' refers to the likelihood of initiation/detonation by accidental means e.g. impact, friction, electrostatic discharge, application of flame, etc.
 It is a quantitative measure of the level of stimulus required to cause initiation, typically by shock/impact.
Epoxy putty is a room-temperature-hardening substance used as a space-filling adhesive.
 It is stored until use as two components of clay-like consistency.
 Kneading the two components into each other creates an exothermic chemical reaction that activates the substance for use by catalyzing an epoxide polymerisation reaction.
 Unlike many other types of glues, an epoxy adhesive can fill gaps and even be molded into a structural part.
 Some makers claim in advertising that one can drill and tap their cured product, and that it quickly cures "hard as steel" (as measured by Shore rating), though it is much weaker than steel in tensile strength and shear strength.
The Explosive Substances Act 1883 (c. 3) is an Act of the Parliament of the United Kingdom.
 It makes it illegal to use (or conspire or intend to use) any explosive substance to cause an explosion likely to endanger life or cause serious injury to property, whether or not any explosion actually takes place.
 A person guilty of an offence under this law is liable to life imprisonment.
Zaur Gurbanli (Azerbaijani: "Zaur Qurbanlı" , born on January 12, 1987, Baku) - is a youth activist, co-founder and ex-board member of N!DA Civic Movement from February 2011 to December 12, 2012, projects coordinator and ex-Senior official for the arrested members` rights in 2013.
 He is known for his arrest on April 1, 2013 a few days after the protests held in Baku against the non-combat deaths in the military.
 Zaur was accused in bogus charges as keeping illegal fire-arm and acquiring explosive substance and devices.
 International human rights organization, Amnesty International recognized him as a prisoner of conscience.
 He was sentenced to 8 years of imprisonment on May 6, 2014, but later was pardoned and released on December 30, 2014 after 1 year 9 months imprisonment.

Question: The explosive substance used by Andrew Kehoe was discontinued for farm use in what year?
Answer: 1928