The main difference between the two is in the way the soap is cooked and processed. In cold process soapmaking, the lye and water are mixed together until the substance saponifies and becomes soap. The curing process then takes 4-6 weeks before a cold-processed soap can be used. In hot-process soapmaking, the lye and water are heated together until it saponifies and becomes soap. But in this case, the hot processed soap can be used immediately after it cools.