The crystallization of epoxy resin comes as a surprise to many. They open up a unit only to find that the resin is as hard as a rock. Often the reaction is “Oh well, it is probably old. It has probably exceeded its shelf-life. I had better buy a new set.” Many times it is unnecessary to buy new because the resin can sometimes be reconstituted with heat.
What has happened is that the resin has crystallized. It has changed phase. It has changed from the liquid phase to the solid phase. The phenomena is similar to water changing from liquid to slush crystals and then to solid ice. This ice can later be returned to liquid by melting it (applying heat), and after being melted it is back to liquid just like it was before freezing. There is no loss of characteristic as water goes through repeated freeze-thaw cycles. The same thing is true for epoxy resins.
Some resins will crystallize faster than others, and cooler storage temperatures often speed the crystallization process. The crystal is slightly more dense than the liquid, so the crystal slowly sinks to the bottom of the container. In a clear glass bottle, the process can be observed. First, the clear looking resin begins to look foggy or milky. As time goes by, a white sludge begins to accumulate at the bottom. This layer will continue to get thicker and thicker until it finally reaches the top. Left undisturbed the crystallization will progress until the whole mass is as solid rock salt. The epoxy resin can be stored indefinitely in the state.
To reliquify the resin apply heat. Put the container in oven set at 150 deg. F. until it is heated throughout, give it an hour or so and by then the crystals will all have melted. Double boilers have also been used successfully. With unfilled resins in plastic bottles, a micro-wave oven can be used. A gallon can usually be micro-waved in 10 to 15 minutes at a medium setting; if the resin still has a milky appearance give it a little more time (just don't melt the plastic bottle). After melting, let the resin cool down to room temperature and use it in the normal way.
With epoxy resins that contain aluminum powder, pigments, or other fillers, there is normally no color change from crystallization. No milkiness can be seen because the resins appearance is controlled by the fillers. Filled crystallized epoxy resin can still be melted by the oven procedure described above, allowed to cool and then used in the normal way. Resins that have been packaged in metal cans or have metal caps should not be micro-waved.