the present invention concerns methods of manufacturing transmission media for telecommunications systems using light radiations and more particularly it relates to a method of and an apparatus for reducing surface and volume defects in silica optical - fibres . as known during drawing both surface and volume defects are generated in the optical fibre . in the former case the defects consist in superficial micro - fractures of different depth which according to their type and density affect more or less seriously the mechanical properties of resistance to traction and dynamic fatigue of the fibre . in the later case they consist in network defects such as micro - crystals , bond defects , etc , which entail the formation of colour centres , i . e . zones wherein the optical radiation is absorbed according to a law dependent on the wavelength . they are generally gaussian curves with the maximum in the ultraviolet band . in addition , the volume defects above raise the value of rayleigh scattering coefficient , and hence the whole attenuation curve . finally , volume defects increase the network reactivity with polluting agents , such as molecular hydrogen , etc , which can diffuse inside giving rise to new absorption zones . not only the optical and geometrical properties of the fibres are to be improved , but also these problems are to be solved . in proc . optical fibre communication washington d . c . march 6 - 8 , 1979 , pages 74 , 75 the paper entitled &# 34 ; improved fatigue resistance of high - strength optical fibres &# 34 ; by r . hiskes , describes an attempt at eliminating at least surface defects by directly applying on the fibre , during drawing and prior to coating it with resin , a thin layer of ceramic material . however surface defects are not completely avoided , as the process cannot take place at high temperature , at which fibre deformations might take place , and hence the adherence between the applied ceramic layer and the fibre is poor . besides , since the deposition is made directly on the fibre , the drawing speed is slowed down and consequently manufacturing times are lengthened . these disadvantages are overcome by the method of reducing volume and surface defects in silica optical - fibres , provided by the present invention , by which surface defects of vitreous silica fibres can be avoided by enriching the surface with a compound which changes its thermo - mechanical properties preventing micro - fractures , without perturbations in chemical equilibrium of material . besides , by small changes to the used apparatus volume defects can be overcome , obtaining a fibre whose vitreous structure better approaches the ideal one . the present invention provides a method of reducing volume and surface defects in silica optical - fibres , characterized in that during the drawing the surface of transition zone of the preform is chemically activated by reducing atmosphere and a layer of carbon particles under nascent state is deposited , making then the carbon particles diffuse by a temperature equal to glass melting temperature and their reaction with silica take place according to the equation where the presence of square brackets means a prevailing silica presence in the preform material , and also characterized in that the just - drawn optical fibre is raised to a temperature equal to or slightly higher than the vitreous transition temperature and hence it is rapidly cooled . the method consists in generating inside the drawing furnace a reducing atmosphere so as to partly activate the surface of the transition zone of the preform , i . e . the zone where the diameter becomes equal to the fibre diameter . simultaneously nascent - state carbon particles are deposited on the upper preform zone , thanks to the thermal - gradient presence . as the preform enters the drawing furnace , the deposited carbon layer meets thermal conditions allowing its weak diffusion inside the material and besides , since the surface is activated by the reducing atmosphere , it can react with the material giving rise to a mixed compound with greater density than silica . hence it results less permeable to polluting agents . carbon as well as hydrogen can be used as reducing agents . surface activating reactions are as follows : where the symbols in square brackets indicate that the material chiefly consists of silica , with a final result of oxygen reduction . the surface activated in presence of nascent carbon can give rise to a second reaction , which takes always place under silica excess conditions and still at drawing temperature (˜ 2000 ° c . ): the result of this second reaction is the partial occupation by carbon atoms of bonds activated by reduction . the final material has a more compact and uniform structure and presents a more regular surface ; as a consequence , its surface properties render it less liable to micro - fractures . besides , since the whole process is based on diffusive phenomena , the material characteristics vary gradually from the inside towards the outside , thus obtaining a less critical structure from the mechanical point of view . the fact of operating at high temperatures , about 2000 ° c ., allows the phenomena above and prevents any discontinuity in the material . to improve material properties from standpoint of volume defects , a rapid - cooling is necessary , i . e . a very high dt / dt ( t = temperature ; t = time ), of the just produced fibre , so as to freeze the molecular state under the conditions of thermal cahos the material has at high temperature , to reproduce the amorphous structure typical of vitreous state . the temperature at which glass formation takes place is the so - named vitreous transition temperature tg . for an ideal glass , this temperature directly determines also the value of rayengh scattering coefficient , and hence optical attenuation properties . usually the fibre rapidly cools as an effect of its reduced thermal capacity ( owing to small dimensions ), passing from drawing temperature to ambient temperature . nevertheless , drawing temperature is gnerally very high , as it corresponds to softening temperature of the material ; more precisely it is about twice as high as the vitreous transition temperature , which for pure silica in equal to 1100 ° c . operating in this way , the cooled material can keep traces of its condition at too high temperature and hence residual attenuation . thus the value of rayleigh scattering coefficient can be higher than the optimal one . the method of the invention provides a passage from softening temperature to ambient temperature through an intermediate step at a temperature slightly higher , for safety purposes , than that of vitreous transition tg . the final product , having been frozen under the best conditions for the glass manufacturing , results volume defectless . reference 1 indicates the preform wherefrom optical fibre 2 is drawn . glass softening temperature , of about 2000 ° c ., is obtained by a furnace 3 , equipped with a silica window 4 , through which the temperature can be optically read . the preform enters the furnace through a pair of disks 5 and 6 , with central holes , between which an inert gas e . g . n 2 or ar , is sent through inlets 7 and 8 . the flow of this gas , sucked by inlets 9 and 10 made in the chamber below , separates the high - temperature zone from the zone outside , communicating with one another through gaps present between preform 1 and disks with central holes 5 and 6 . methane ( ch 4 ) is injected through inlets 11 and 12 ; thanks to high temperature present in furnace 3 , it decomposes into c 2 and h 2 . nascent state carbon and an atmosphere rich in hydrogen , and consequently with reducing properties , are thus obtained . c 2 and h 2 flow impinges onto the preform , where c 2 deposits according to normal thermophoretic phenomena and is sucked through inlets 9 and 10 . the just - obtained fibre traverses a second furnace 13 , capable of eusuring at its inside a slightly higher temperature than vitreous transition temperature , e . g . 1120 ° c . also this furnace is provided with a window 14 , through which fibre diameter can be read and is closed at the bottom by a disk 15 with central hole . an inert gas is injected through inlets 16 and 17 inside the furnace 13 . said gas is preheated at 1120 ° c . and has the task of sealing the gap present between disk 15 and optical fibre 2 . after traversing disk 15 , the fibre enters a chamber at low temperature , whose value ranges between + 20 ° and - 50 ° c ., delimited by a conditioner 20 and a pair of disks 23 and 26 with central holes . a cold inert gas is injected into the chamber through inlets 18 and 19 and is then sucked away through inlets 22 and 21 . an inert gas is injected also through inlets 24 and 25 with the task of sealing the gap comprised between disk 26 and the fibre . it is clear that what described has been given by way of non limiting example . modifications and variations are possible without going out of the scope of the invention .