A method of cleaning silica-based soot or a porous soot preform, said method comprising: treating said silica-based soot or said a porous soot preform with at least one of the following compounds: (i) a mixture of CO and Cl 2 in a carrier gas such that the total concentration of CO and Cl 2 in said mixture is greater than 10%, by volume and the ratio of CO:Cl 2 is between 0.5 and 2, wherein the treatment time with the CO/Cl 2 mixture is at least: t treatment, Cr 2 O 3 in min > t diffusion + t reaction, Cr 2 O 3 where the diffusion reaction time is a function of soot layer thickness L (in cm) and diffusion rate of the CO/Cl 2 mixture D eff (in cm 2 /sec) through the porous soot preform, or loose silica soot and t diffusion in min = L 2 60 D eff; t diffusion;and the reaction time is t reaction, Cr 2 O 3 in min = 4.3 × 10 − 4 d p in μm Exp 12000 / T in K y Cl 2 x Cl 2 1 − 1.35 x Cl 2 + 0.372 x Cl 2 2 where X Cl2 is (y Cl 2)/(y Cl 2 + y CO), y cl2 and y co are the partial pressure of chlorine and carbon monoxide respectively and d p is the cross-sectional length or diameter of Cr 2 O 3 particles present in the silica soot or the porous soot preform; (ii) CCl 4, in a carrier gas, such that the concentration of CCl 4 is greater than 1%, by volume, wherein the treatment with CCl 4 is performed between 600 °C and 850 °C, and wherein the treatment time with CCl 4 is at least: t treatment, ZrO 2 in min > t diffusion + t reaction, ZrO 2 where the diffusion reaction time is a function of soot layer thickness L (in cm) and diffusion rate of the CCl 4 D eff,CCl4 (in cm 2 /sec) through the porous soot preform and is t diffusion in min = L 2 60 D eff, CCl 4 and the reaction time is given as: t reaction, ZrO 2 in min = 5.75 × 10 − 6 d p in um Exp 12000 / T y CC 14 in atm, wherein d p is the cross-sectional length or diameter of ZrO 2 particles present in the silica soot or the porous soot preform. (i) a mixture of CO and Cl 2 in a carrier gas such that the total concentration of CO and Cl 2 in said mixture is greater than 10%, by volume and the ratio of CO:Cl 2 is between 0.5 and 2, wherein the treatment time with the CO/Cl 2 mixture is at least: t treatment, Cr 2 O 3 in min > t diffusion + t reaction, Cr 2 O 3 where the diffusion reaction time is a function of soot layer thickness L (in cm) and diffusion rate of the CO/Cl 2 mixture D eff (in cm 2 /sec) through the porous soot preform, or loose silica soot and t diffusion in min = L 2 60 D eff; t diffusion;and the reaction time is t reaction, Cr 2 O 3 in min = 4.3 × 10 − 4 d p in μm Exp 12000 / T in K y Cl 2 x Cl 2 1 − 1.35 x Cl 2 + 0.372 x Cl 2 2 where X Cl2 is (y Cl 2)/(y Cl 2 + y CO), y cl2 and y co are the partial pressure of chlorine and carbon monoxide respectively and d p is the cross-sectional length or diameter of Cr 2 O 3 particles present in the silica soot or the porous soot preform; (ii) CCl 4, in a carrier gas, such that the concentration of CCl 4 is greater than 1%, by volume, wherein the treatment with CCl 4 is performed between 600 °C and 850 °C, and wherein the treatment time with CCl 4 is at least: t treatment, ZrO 2 in min > t diffusion + t reaction, ZrO 2 where the diffusion reaction time is a function of soot layer thickness L (in cm) and diffusion rate of the CCl 4 D eff,CCl4 (in cm 2 /sec) through the porous soot preform and is t diffusion in min = L 2 60 D eff, CCl 4 and the reaction time is given as: t reaction, ZrO 2 in min = 5.75 × 10 − 6 d p in um Exp 12000 / T y CC 14 in atm, wherein d p is the cross-sectional length or diameter of ZrO 2 particles present in the silica soot or the porous soot preform.