These animal glue companies apparently used mostly waste by-products from tanneries, unlike the common story about old horses going to the glue factory. While they may have used some horses that was a very minor source of material.
My previous project of building a model tannery was not all that different from the glue factory. They employed similar processes. the glue factory had three major operations; preparing the stock, boiling the stock and treating and drying the resulting glue solution. The glue making process was very messy as seen in the photo below.
I have made a rough design and started on the first building in the complex. Doing the initial design is always difficult, second only to deciding on what I will model. The inspiration postcard is from a moderately large collection of railroad and industry postcards I collected over the years. During the late 1800s and the first half of the 1900s postcards were a popular means of communication. Now we have email and texting, and for some people (not me) social media where people share way too much information. I find some of the messages on old postcards to be interesting insights into peoples lives - sort of like being able to read peoples emails, something hackers occasionally do. The Internet, while quite useful, can be a dangerous place if you are not careful. That is why I do not post any comments that include Internet links. While most are probably innocuous, I do not want to take a chance nor should I put any reader in that position. We are now also entering on the era of the Internet of Things (IOT). That will include situations where, for example, your refrigerator can communicate with you. I don't need my appliances sending me messages nor do I need to send messages to them. But before you decide I just don't understand these things, I spent a significant part of my working life in the computer and information security business.