Sludge metal
Sludge metal (also known as sludge or sludge doom) is an extreme subgenre of heavy metal music that originated through combining elements of doom metal and hardcore punk. It is typically harsh and abrasive, often featuring shouted vocals, heavily distorted instruments, and sharply contrasting tempos. The Melvins from the US state of Washington produced the first sludge metal albums in the mid-late 1980s.

Characteristics
The key characteristics of both sludge and doom metal are a slow tempo combined with down-tuned, heavily distorted guitars to deliver an extremely heavy feel. It can be hard to tell the difference between doom, stoner, and sludge metal because of the similarities between the three genres. Sludge metal is distinguished primarily by its use of elements of hardcore punk, such as bursts of faster-paced material, and its avoidance of the psychedelic rock elements that distinguish stoner metal. Additionally, sludge metal incorporates feedback and noise elements from industrial bands like SPK, Throbbing Gristle, and Swans.Sludge metal is also distinguishable from doom metal by its themes and tone. Doomed to Fail author J. J. Anselmi writes that sludge is more likely to explore real-world themes like poverty and addiction, as opposed to the "epic battle between good and evil" often portrayed in doom metal lyrics. "Sludge is typically more grounded, delving into life's negativity while laughing at its absurdity," Anselmi writes. Eyehategod vocalist Mike IX Williams says the term sludge relates to "the slowness, the dirtiness, the filth,  and general feel of decadence the tunes convey".
Sludge metal is regarded as particularly challenging for drummers to play because they must be able to lead the band through the slow parts of a piece with an accurate time feel, which is much harder to achieve than when playing faster pieces.

History
Initially, the Melvins played hardcore punk, but, according to Krist Novoselic (later the bassist of Nirvana), frontman Buzz Osborne was inspired to change directions after seeing the hardcore punk band Black Flag during their 1984 tour. By the time Black Flag had moved away from their faster hardcore punk roots and were playing slower, sludgier, more Black Sabbath-inspired music, as heard on their album My War. According to Novoselic, Osbourne began writing "slow and heavy riffs" to form a dirge-like music that was the beginning of both sludge metal and northwest grunge. Another band that combined punk with the heaviness of Black Sabbath was Flipper, who also influenced Melvins and other early sludge bands like Eyehategod.The Melvins' Six Songs (1986) and Gluey Porch Treatments (1987) are regarded as the first post-punk sludge albums. The remnants of hardcore punk mixed into the songs of Gluey Porch Treatments provides one of the key differences that distinguish sludge from doom metal. According to Pantera and Down vocalist Phil Anselmo: "Back in those days, everything in the underground was fast, fast, fast. It was the rule of the day...But when the Melvins came out with their first record, Gluey Porch Treatments, it really broke the mold, especially in New Orleans. People began to appreciate playing slower."The early development of grunge and sludge metal were closely linked. Nirvana's single "Love Buzz/Big Cheese" (1988) was described by their record label Sub Pop as "heavy pop sludge". The Seattle music author and journalist Gillian G. Gaar writes that Nirvana's debut studio album Bleach "does have its share – some would say more than its share – of dirty sludge". Examples of crossover tracks between sludge and grunge include Soundgarden's "Slaves & Bulldozers" (1991) and Alice in Chains' "Sludge Factory" (1995). The American music author and journalist Michael Azerrad described the short-lived Seattle band Blood Circus (1988) as a sludge metal band, with rock journalist Ned Raggett describing the band's music as "rough and ready, sludgy guitar rock with a bad attitude".In an article describing the sound of The Melvins, the The New York Times wrote: "The shorthand term for the kind of rock descending from early Black Sabbath and late Black Flag is sludge, because it's so slow and dense." Metal Hammer cited Black Sabbath's "downcast metal," Black Flag's "tortured hardcore," and the "sub/dom grind of early Swans" as influences on the genre.Other key bands in the development of sludge metal include Acid Bath, Buzzoven, Corrupted, Crowbar, Down, Eyehategod, and Grief. By the late 1990s, small sludge scenes could be found across many countries.By the early 2000s, sludge metal had formed cross-over works with stoner metal, such as Bongzilla's "Gateway" and High on Fire's "The Yeti".

Sludgecore
Eyehategod formed in Harvey, Louisiana, in 1988 and is sometimes credited with originating a new style sometimes referred to as "New Orleans hardcore-edged sludge" or "sludgecore,"  though some see sludgecore as a movement that emerged in New Orleans with no single originator. Other bands regarded as sludgecore include Acid Bath, Crowbar, Eyehategod, and Soilent Green.Like sludge metal, sludgecore combines sludge metal with hardcore punk, and possesses a slow pace, a low guitar tuning, a grinding dirge-like feel. some faster tempo songs or passages, as heard on Eyehategod's debut album In the Name of Suffering The guitars (electric guitar and bass guitar) are often played with large amounts of feedback. Vocals are usually shouted or screamed. Some sludgecore bands are influenced by Southern rock. Lyrics by sludgecore bands like Crowbar tend to be bleak in nature. Buzzoven's lyrics concern drug abuse, and Acid Bath's concern rape, abortion, death, and self-loathing.

See also
List of sludge metal bands
Crust punk

References
Publications
Bukszpan, Daniel (2012). The Encyclopedia of Heavy Metal. Sterling, New York. ISBN 978-1402792304.