A method of manufacture of man-made vitreous fibres (MMVF) comprising: providing a fiberising apparatus, wherein the fiberising apparatus comprises: a set of at least four rotors each mounted for rotation about a different substantially horizontal axis; wherein each rotor has a driving means; rotating the rotors; wherein the first rotor rotates to give an acceleration field in the range 25 to 60 km/s 2, the second rotor rotates to give an acceleration field in the range 125 to 250 km/s 2, the third rotor rotates to give an acceleration field in the range 150 to 300 km/s 2 and the fourth rotor rotates to give an acceleration field in the range 225 to 400 km/ 2, wherein the acceleration field of the last rotor is from 5 to 15 times greater than the acceleration field of the first rotor, providing a mineral melt; wherein the melt has a composition comprising the following, expressed by wt of oxides: SiO 2 in an amount of from 33 to 45 wt%, Al 2 O 3 in an amount of from 16 to 24 wt%, an amount of K 2 O and/or Na 2 O, an amount of CaO and/or MgO, wherein the ratio of the amount of Al 2 O 3 to the amount of SiO 2 is in the range 0.34-0.73, wherein the ratio of the total amount of K 2 O and Na 2 O, to the total amount of CaO and MgO, is less than 1, pouring the melt on to the periphery of the first rotor; wherein melt poured on to the periphery of the first rotor in the set is thrown on to the periphery of the subsequent rotors in turn and fibres are thrown off the rotors; and collecting the fibres that are formed. providing a fiberising apparatus, wherein the fiberising apparatus comprises: a set of at least four rotors each mounted for rotation about a different substantially horizontal axis; wherein each rotor has a driving means; rotating the rotors; wherein the first rotor rotates to give an acceleration field in the range 25 to 60 km/s 2, the second rotor rotates to give an acceleration field in the range 125 to 250 km/s 2, the third rotor rotates to give an acceleration field in the range 150 to 300 km/s 2 and the fourth rotor rotates to give an acceleration field in the range 225 to 400 km/ 2, wherein the acceleration field of the last rotor is from 5 to 15 times greater than the acceleration field of the first rotor, wherein the first rotor rotates to give an acceleration field in the range 25 to 60 km/s 2, the second rotor rotates to give an acceleration field in the range 125 to 250 km/s 2, the third rotor rotates to give an acceleration field in the range 150 to 300 km/s 2 and the fourth rotor rotates to give an acceleration field in the range 225 to 400 km/ 2, wherein the acceleration field of the last rotor is from 5 to 15 times greater than the acceleration field of the first rotor, providing a mineral melt; wherein the melt has a composition comprising the following, expressed by wt of oxides: SiO 2 in an amount of from 33 to 45 wt%, Al 2 O 3 in an amount of from 16 to 24 wt%, an amount of K 2 O and/or Na 2 O, an amount of CaO and/or MgO, wherein the ratio of the amount of Al 2 O 3 to the amount of SiO 2 is in the range 0.34-0.73, wherein the ratio of the total amount of K 2 O and Na 2 O, to the total amount of CaO and MgO, is less than 1, SiO 2 in an amount of from 33 to 45 wt%, Al 2 O 3 in an amount of from 16 to 24 wt%, an amount of K 2 O and/or Na 2 O, an amount of CaO and/or MgO, wherein the ratio of the amount of Al 2 O 3 to the amount of SiO 2 is in the range 0.34-0.73, wherein the ratio of the total amount of K 2 O and Na 2 O, to the total amount of CaO and MgO, is less than 1, pouring the melt on to the periphery of the first rotor; wherein melt poured on to the periphery of the first rotor in the set is thrown on to the periphery of the subsequent rotors in turn and fibres are thrown off the rotors; and collecting the fibres that are formed.