If the house was built prior to the 1920s it probably was horse glue. Once the horses got replaced by cars they had to find other glues. In our rental properties we always used a heat gun and sc****r. It works well but there is some fire danger especially when you get to wall area. We kept a small water sprayer handy and presprayed all wall/floor corners and then cooled it off with the sprayer immediately after scraping each area.