610 ILCS 5/13
The directors of such corporation may require the subscribers to the capital stock of such corporation to pay the amount by them respectively subscribed, in such manner and in such installments as they may deem proper. If any stockholder shall neglect to pay any installment as required by a resolution or order of such board of directors, the said board shall be authorized to declare such stock and all previous payments thereon forfeited for the use of the corporation; but the said board of directors shall not declare such stock so forfeited until they shall have caused a notice in writing to be served on such stockholder personally, or by depositing the same in a post office, properly directed to the post office address of such stockholder, or if he be dead, to his legal representatives, with necessary postage for its transmittal properly prepaid, stating therein that in accordance with such resolution, or order, he is requested to make such payment, at a time and place and in the manner to be specified in such notice, and that if he fails to make the same in the manner requested, his stock and all previous payments thereon will be forfeited for the use of such corporation; and thereafter such corporation, should default in payment be made, may sell the same and issue new certificates of stock therefor: Provided, that the notice aforesaid shall be personally served or duly deposited, as above required, at least sixty days previous to the day on which such payment is required to be made.
610 ILCS 5/13