Rosin-cores
The Encyclopedia of Founding (1892)
When a core contains more or less rosin in its composition it is called a rosin-core. Where large numbers of dry-sand column or other cores are in constant requisition, rosin may be readily substituted for flour if a pulverizer is obtained for grinding the cheap grades bought in bulk; besides which it is much cheaper than good foundry flour. To secure the best results, it is important that the rosin be ground very fine in order that its gumminess may be more generally disseminated throughout the mass, and thus strengthen the green core. It may also be said that, owing to the closer intimacy of the grains of rosin, the sand-grains are spread out and a free passage is made for the gases to travel towards the vents. For small cores made from fine beach or free sand the proportion of rosin may be one to eleven; less when stronger sands are used. Should cores made from this proportion lack stiffness when green, a little molasses or glue-water will serve to increase their tenacity. Large column-cores, round or square, may be made from a mixture composed of 14 each of fire and beach sand, with 6 of moulding-sand and 3 of finely pulverized rosin added. Cores made from these ingredients, if well dried and allowed to cool before removing, are extremely tough and unyielding, and for this reason the system of core-ironing may be of the simplest kind. The sands composing these mixtures being principally free sand, are at once liberated when the rosin has burned out, making the core-cleaning a matter of the least difficulty imaginable. See FLOUR; MOLASSES; GLUE; CORE-SAND.
What Has Changed in the Last Century
When the Encyclopedia of Founding described rosin-cores in 1892, core strength came from cheap ground rosin, flour, molasses and glue-water, and the recipes were mixed by eye in the foundry. Those organic binders were displaced in the twentieth century by synthetic resins: Johannes Croning's shell-moulding process, patented in Germany in the 1940s, bonded sand with phenolic resin baked against a heated pattern, and the sodium-silicate/CO2 and amine-cured cold-box processes that followed let cores harden in seconds without a drying oven. Modern foundries buy pre-coated sand to specification rather than pulverising bulk rosin, and cores for complex castings are increasingly printed directly by binder-jet 3D machines. The practical concerns the 1892 writer names — gas escaping to the vents, and a core that collapses easily for cleaning — are still exactly what a core binder is judged on, and today the phrase "rosin core" is more likely to mean flux-cored solder than a foundry core.
Written for this edition, 2026.