Sludge metal generally combines the slow tempos, heavy rhythms and dark, pessimistic atmosphere of doom metal with the aggression, shouted vocals and occasional fast tempos of hardcore punk. As The New York Times put it, "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." According to Metal Hammer, sludge metal "[s]pawned from a messy collision of Black Sabbath’s downcast metal, Black Flag’s tortured hardcore and the sub/dom grind of early Swans, shaken up with lashings of cheap whisky and bad pharmaceuticals". Many sludge bands compose slow-paced songs that contain brief hardcore passages (for example, Eyehategod's "Depress" and "My Name Is God"). Mike Williams, a founder of the sludge style and member of Eyehategod, suggests that "the moniker of sludge apparently has to do with the slowness, the dirtiness, the filth and general feel of decadence the tunes convey". However, some bands emphasize fast tempos throughout their music. The string instruments (electric guitar and bass guitar) are down-tuned and heavily distorted and are often played with large amounts of feedback to produce a thick yet abrasive sound. Additionally, guitar solos are often absent. Drumming is often performed in typical doom metal fashion. Drummers may employ hardcore d-beat or double-kick drumming during faster passages, or through the thick breakdowns (which are characteristic of the sludge sound). Vocals are usually shouted or screamed, and lyrics are generally pessimistic in nature. Suffering, drug abuse, politics and anger towards society are common lyrical themes.
Many sludge metal bands from the Southern United States incorporate Southern rock influences, although not all sludge bands share this style. There is some controversy as to whether the term refers to only the style emerging from New Orleans and later the American South more broadly, or to "a complete consciousness in the heads of like-minded Black Flag/Black Sabbath influenced scenes and individuals all over the world". So-called "atmospheric" sludge bands adopt a more experimental approach and compose music with an ambient atmosphere, reduced aggression and philosophical lyrics. Due to the similarities between sludge and stoner metal, there is often a crossover between the two genres, but sludge metal generally avoids stoner metal's usage of psychedelia. Sludge metal also bears some musical and lyrical resemblance to crust punk, due to the usage of political lyrics and thick, "dirty" guitar sounds.
Perhaps the most significant influence was Melvins, a band from the state of Washington. Their earliest releases, Six Songs (1986) and Gluey Porch Treatments (1987), are often regarded as the first sludge records. At this time, the band was also an important member of the Washington grunge scene. Another prominent band from the Washington grunge scene, Alice in Chains have also been influential to early sludge metal with their second album Dirt.Neurosis, from Oakland, were also significant early practitioners.
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. With that, all the old Black Sabbath came back around and then you start digging and you come to your Saint Vitus, your Witchfinder General, your Pentagram, etc.
Sludge metal subsequently spread throughout the Southern and Eastern United States.
Jose Carlos Santos notes a focus shift as a result of the impact of the British group Iron Monkey's first album in 1997:
Coincidence or not, it seemed like the sludge floodgates opened to the rest of the world, and in the past decade small pockets, or mini-scenes, can be spotted in just about any country you'd care to mention.
During the late 1990s, many sludge metal bands began to incorporate post-rock elements into their music. This post-rock/sludge crossover was greatly inspired by the experimental style of Neurosis during the early to mid 1990s, and is performed by prominent bands such as Isis,Cult of Luna and Pelican.
These bands are the pioneers of the genre or are strongly influenced by those bands. Many are from the Southern United States, excluding the Melvins who are from Montesano, Washington and Iron Monkey who are from England.
^ abMason, Stewart. "Kylesa". AllMusic. Retrieved 2008-09-02. …elements of hardcore punk, psychedelic stoner rock, technical speed metal, and good old-fashioned Black Sabbath sludge appear in their music.
^Rivadavia, Eduardo & Koets, Tara. "Electric Wizard". AllMusic. Retrieved 2008-09-02. …it so effortlessly bridged the stylistic gaps between doom, sludge, stoner, horror, and, at times, even space metal…