Calcium hydroxide is made by adding water to calcium oxide (quicklime), which itself comes from heating calcium carbonate (limestone, eggshells, or chalk) to high temperatures. The full process has two stages: first you burn your calcium source to create quicklime, then you hydrate that quicklime with water. The chemistry is straightforward, but the details of temperature, water ratio, and safety matter a lot for getting a usable product.
The Two-Step Chemistry
Every method of making calcium hydroxide follows the same pair of reactions. In step one, you heat calcium carbonate to drive off carbon dioxide, leaving behind calcium oxide. In step two, you add water to that calcium oxide, which reacts vigorously and produces calcium hydroxide plus significant heat. The first reaction requires sustained high temperatures. The second happens almost instantly once water contacts the powder.
Step 1: Burning Calcium Carbonate Into Quicklime
Your starting material needs to be rich in calcium carbonate. Limestone is the traditional source, but crushed eggshells, oyster shells, and chalk all work because they’re chemically similar. Eggshells calcined at 800°C for one hour yield calcium oxide with 97 to 98% purity, according to lab testing in rotary kilns. Limestone generally needs temperatures in the 850 to 900°C range for complete conversion, though some sources calcine successfully at 800°C.
In practical terms, this means you need a kiln, furnace, or similar setup capable of reaching and holding at least 800°C (about 1,470°F). A standard backyard fire tops out around 600 to 700°C, which isn’t enough. Potters’ kilns, propane-fired forges, and purpose-built lime kilns can reach the necessary range. Break your material into small, roughly uniform pieces before firing so heat penetrates evenly. Hold the temperature for at least an hour to ensure the carbon dioxide has fully escaped.
You’ll know calcination is complete when the material has lost roughly 44% of its weight (that’s the CO₂ leaving), turned lighter in color, and become noticeably more porous and crumbly. The resulting quicklime is highly reactive, so handle it with care and move to step two relatively soon.
Step 2: Adding Water to Quicklime
This hydration step is where calcium hydroxide actually forms. You have two approaches depending on what form you want the final product in.
Dry hydration uses just enough water to react with the quicklime without leaving excess moisture. The quicklime absorbs the water, heats up dramatically (temperatures can spike above 150°C), and crumbles into a fine, dry white powder. The typical ratio is roughly 1 part water to 2.5 to 3 parts quicklime by weight, though the exact amount depends on the purity of your quicklime. Add water gradually and in small amounts, stirring or mixing as you go. The powder that results is what’s sold commercially as hydrated lime or slaked lime.
Wet slaking uses excess water, producing a thick slurry sometimes called milk of lime. This is the easier method if you don’t need a dry powder. Place your quicklime chunks in a heat-resistant, non-metallic container and slowly pour water over them. The reaction will bubble, steam, and generate intense heat. Keep adding water until the mixture becomes a smooth paste or thin slurry, then let it settle. The calcium hydroxide will sink to the bottom. You can decant the water and dry the paste if you want a powder, or use the slurry directly.
Choosing Your Starting Material
Limestone from a quarry or building supply store is the cheapest and most common source for large batches. Its purity varies, so construction-grade limestone will contain more impurities (clay, silica, iron) than chemical-grade material. If you’re making calcium hydroxide for food preparation, pickling, or nixtamalization, the purity of your source matters significantly.
Eggshells are a surprisingly effective alternative for small batches. They’re nearly pure calcium carbonate once you wash off the membrane. Collect shells, remove the inner membrane, wash thoroughly, dry them, and crush before calcining. Research on eggshell-derived calcium hydroxide shows it performs comparably to commercial products for applications like water treatment.
Food Grade vs. Industrial Grade
If you plan to use calcium hydroxide for anything involving food or drinking water, purity standards are strict. Food-grade calcium hydroxide must contain at least 95% pure product, with arsenic below 3 parts per million, lead below 2 ppm, and fluoride below 0.05%. These limits are set by the Food Chemicals Codex and enforced by the FDA. Construction or fertilizer-grade lime doesn’t need to meet these thresholds and often contains higher levels of heavy metals and other impurities.
For food applications like making tortillas, treating corn, or preparing betel leaf, buying certified food-grade calcium hydroxide is far safer than making your own. Homemade batches have no way to verify heavy metal content without lab testing.
Safety During Preparation
Both quicklime and calcium hydroxide are caustic. A saturated solution of calcium hydroxide in water has a pH of about 12.5, making it strongly alkaline. The dry powder causes skin irritation on contact and can cause serious, potentially blinding eye damage. Inhaling the dust irritates your airways and can trigger coughing and difficulty breathing.
Wear chemical-splash goggles (not just safety glasses), heavy-duty gloves, long sleeves, and a dust mask or respirator rated for fine particulates. Work outdoors or in a well-ventilated area. The slaking reaction produces steam that carries fine caustic particles, so stand upwind and never lean over the container. Keep a large supply of clean water nearby in case of skin or eye contact.
Storing Calcium Hydroxide
Calcium hydroxide gradually reacts with carbon dioxide in the air, converting back into calcium carbonate, which is essentially chalk. This process, called carbonation, starts within about 15 days of exposure and can completely convert your product to useless calcium carbonate within 90 days. Higher humidity accelerates this dramatically.
Store your finished calcium hydroxide in airtight containers with minimal headspace. Glass jars with tight-sealing lids or heavy-duty zip-seal bags with the air squeezed out both work. Keep it in a cool, dry location. If you notice the powder becoming less reactive or less alkaline over time, carbonation has likely degraded it.
Solubility in Water
Calcium hydroxide dissolves poorly in water. At 25°C, only about 1.5 grams dissolve per liter, producing a clear solution called limewater with a pH of 12.45. Counterintuitively, it dissolves slightly better in cold water than warm water: solubility is about 4% higher at 20°C and 4% lower at 30°C. This reverse solubility means heating the solution won’t help you dissolve more.
If you need a stronger alkaline solution than limewater can provide, you’ll work with a slurry (undissolved calcium hydroxide suspended in water) rather than a true solution. For uses like adjusting soil pH, treating water, or making whitewash, the slurry form is perfectly effective. Stir or shake it before each use since the particles settle quickly.