Work on In Rock began shortly after Gillan and Glover joined the band in June 1969, with rehearsals at Hanwell Community Centre.

The album has continued to attract critical praise as a key early example of the hard rock and heavy metal genres.

[5][6] Although the group contained experienced musicians, none of the original members were accomplished songwriters,[7] and their earlier work ranged from psychedelic hard rock based on guitarist Ritchie Blackmore's riffs, classical-influenced tracks developed and arranged by organist Jon Lord, and cover songs from the Beatles, Joe South, Neil Diamond and Donovan.

[5][8] After a US tour in May, Blackmore, Lord and drummer Ian Paice decided to replace original lead singer Rod Evans with someone who could tackle a hard rock style.

[5] The group had also recently signed a deal with Harvest Records in the UK, who were intending to represent progressive and underground bands, but label owner Malcolm Jones thought Deep Purple relied too much on gimmicks and only appealed to the US market.

[18] Hanwell Community Centre was booked for the band to rehearse and write new material; according to Glover, it was chosen because "it was the only place we could find where we could make a lot of noise".

[19] The basic structure of "Child in Time" was worked out at these sessions, and both it and "Speed King" (then titled "Kneel and Pray") were in the live set by the line-up's seventh gig at the Paradiso, Amsterdam, on 24 August.

Though the concerto was a different style to the material worked on at Hanwell, it led to increased publicity in the UK, which along with the group's live act, started to give them a following.

[25] The basic ethos behind recording is that everything had to be loud and heavy; Glover recalls seeing VU meters in the red (signalling audio distortion) in the studio.

[27] The album was the first one produced by the group, though they made prominent use of the engineers at the sessions, particularly Martin Birch who aimed to reproduce the live sound of the studio room on tape.

[29] The cover was designed by the group's management; it depicts Mount Rushmore with photographs of the band's faces superimposed over the US presidents.

[25] "Speed King" developed from a bass riff written by Glover at Hanwell, in an attempt to emulate Jimi Hendrix's "Fire".

[1] The song would be re-recorded 28 years later, with Steve Morse on guitar, and retitled "Bludsucker" for Deep Purple's 1998 album Abandon.

[34][35] "Child in Time" was written early during the Hanwell rehearsals, after Lord began playing the introduction to "Bombay Calling" by It's a Beautiful Day.

The group decided to play the song's main theme at a slower tempo, with Gillan writing new words inspired by the Vietnam War.

[40] After completing the album, the group's management were worried there was no obvious hit single, and booked De Lane Lea in early May 1970 so the band could write and record one.

After struggling to come up with a commercial-sounding song, Blackmore started playing the riff to Ricky Nelson's arrangement of "Summertime", while the group improvised the rest of the structure.

[42] The original release featured a gatefold sleeve with full lyrics, and a set of black and white photographs of the band.

In October, while touring the UK, Melody Maker ran a feature of "Purple Mania" showing the group's concerts were attracting increasingly enthusiastic crowds.

The album was remastered by Glover, adding "Black Night", "Jam Stew", a new mix of "Cry Free", and remixes of "Flight of the Rat" and "Speed King".

[42] In 2009 audiophile label Audio Fidelity released a remastered version of Deep Purple in Rock on a limited edition 24 karat gold CD.

Record Mirror's Rodney Collins said it was "a stunningly good album" showing that "rock, given a fresh stab and alert material, is still one of the most rewarding areas of contemporary music.

"[29] Richard Green, writing in New Musical Express, said the album was "Good, meaty rock all the way" and particularly praised Gillan's singing on "Child In Time".

[29] Disc and Music Echo rated it 4 stars out of 5, comparing the sound to the Nice, and noting Blackmore's instrumental dominance over Lord.

"[50] Sid Smith remarked in his BBC Music review the "strident confidence that the new line-up had found" and how the album "pretty much carved out the template for heavy rock.