24 the cooling fluid 21 passes from the tubes at exits 25 and 26 the rate of flow of the cooling fluid 21 through the cooling tube 17 is controlled by valves 23 and 24 this rate is selected at such a value as to carry away heat in sufficient amounts to allow a layer of solidified nutrient 27 to form around the outside of the cooling tubes 17 this layer of solid nutrient 27 formed around the cooling tubes has three functions first it physically isolates the material of the cooling tubes from the nutrient melt to avoid contamination of the ultra pure nutrient melt second it thoroughly insulates the cooling tubes from the melt which would normally be at a temperature far in excess of the melting point of the material from which the tubes are made the flow of coolng fluid in the masking system would be adjusted so that the melting temperature of the material from which the cooling tubes are made occurs somewhere within the solid nutrient layer rather than within the cooling tubes third by adjusting the flow of cooling fluid through the masking system the thickness of the solid nutrient layer can be adjusted and is.