A solution sometimes called water glass. Sodium silicate. I'm not a fan of it. It changes the characteristics of the egg. But it does work. Easier to have chickens and electric lights during the winter to keep them laying.
That technique was primarily used when chickens were raised during spring through fall, and most slaughtered for winter, with a few left over to make a new flock the next year.
/johnny
RE: Chickens... Grandma lived up on a mountain and had chickens who laid all thru the winter. She took all her leftovers, boiled them into slop and fed that stuff to them, along with their feed. She swore that was the reason she had eggs year round. Lord, I wish I still had her around to teach me all she knew...