Where a person has used in part the material which belonged to him and in part that which did not belong to him to form a thing of a new kind, without either of the two materials being entirely destroyed, but in such a way that they cannot be separated without inconvenience, the thing is common to the two owners, in proportion, as regards the one, to the material which belonged to him, as regards the other, in proportion both to the material which belonged to him and to the price of his labour. The price of the labour is estimated on the date of the auction provided for in article 575.