It shall be the duty of the state department in cooperation with the state department of health to establish reasonable minimum standards for foster-home care to which all certified foster homes must conform. No unsupervised foster home shall be certified until an investigation of the home and its standards of care has been made by the state department or by a licensed child welfare agency serving as its representative. Any such home that conforms to the established standards of care and to the prescribed rules shall receive a certificate from the state department, which shall be in force for one year from the date of issuance and which may be renewed unless revoked because of wilful violation of the provisions of this chapter. The certificate shall show the name of the persons authorized to conduct the home, its exact location and the number of children that may be received and cared for at one time. No certified foster home shall receive for care more children than are specified in the certificate.