It is an ammonia compound and not harmful after baking. However, you can't eat the raw dough and the kitchen will stink of ammonia while the cookies bake. Make sure your mixer is up to the job of these cookies. I burned out the motor on my mother's Sunbeam. Faced with a dearth of springerle at Christmas, my husband bought me a new mixer for Thanksgiving.
Flat molds are easier than the rolling pin molds.
The rolling pin molds distort the dough and the picture. Older molds and imported molds have better, sharper pictures. The House on the Hill also sells Hartshorn, pre-made dough, and flavorings. Do not double this recipe. Gradually beat in sugar, keep on beating, by hand one hour, one-half hour by mixer.