Measurements of superconducting properties such as bulk and surface critical fields and thermal conductivity have been carried out in the temperature range from 2 K to 8 K on  large-grain samples of different purity and on a high-purity fine-grain sample, for comparison. The samples were treated by electropolishing and low temperature baking (120 °C, 48 h). While the residual resistivity ratio changed by a factor of ˜3 among the samples, no significant variation was found in their superconducting properties. The onset field for flux penetration at 2 K, H_ffp,, measured within a˜30 μm depth from the surface, was ˜160 mT, close to the bulk value. The baking effect was mainly to increase the field range up to which a coherent superconducting phase persists on the surface, above the upper critical field.