Because an additional agent is required to complete the resin's cure, the industry commonly refers to novolac resins as "two-stage" or "two-step" products. The most common phenolic resin cross-linking agent is hexamethylenetetramine, also known as hexa, hexamine, or HMTA. Ground and blended with the resin, hexa serves as a convenient source of formaldehyde when heated to molding and curing temperatures. A special attribute of hexa is that it reacts directly with resin and phenol without producing appreciable amounts of free formaldehyde. Hexa cures the resin by further linking and polymerizing the molecules to an infusible state. Due to the bond angles and multiple reaction sites involved in the reaction chemistry, the resulting polymer is not a long straight chain but rather a complex three-dimensional polymer network of extreme molecular weight. This tightly cured bonding network of aromatic phenolics accounts for the cured materials' hardness, and heat and solvent resistant properties.