Water Vessels

Part of Pottery and Ceramics

Making watertight pottery for water storage — from sealing porous earthenware to designing vessels that keep water cool, clean, and accessible.

Why This Matters

Clean water storage is a survival essential. Without sealed containers, collected water evaporates, becomes contaminated with insects and debris, or grows algae and bacteria within days. In a rebuilding scenario, you need vessels that hold water reliably for drinking, cooking, food preservation (brining, fermenting), and chemical processes (tanning, dyeing, soap-making).

Pottery is the oldest and most practical solution for water storage. Unlike wood (which rots), leather (which leaks and molds), or metal (which is scarce and corrodes), fired clay is chemically inert, impervious to biological decay, and made from one of the most abundant materials on Earth. A single well-made water jar can serve a household for decades.

The challenge is that basic earthenware — pottery fired below 1,100C — is porous. It seeps. A standard low-fired pot loses 5-15% of its water content per day through the walls. For short-term use this is fine, but for storage it is unacceptable. This article covers the techniques for making truly watertight vessels at any firing temperature.

Designing Water Vessels

Shape Principles

Water vessel design follows functional logic:

• Narrow mouth: Reduces evaporation, keeps debris and insects out, and makes the vessel easier to seal with a lid or cloth. The opening should be just wide enough to fill and pour from — 8-15 cm diameter for household vessels.
• Wide body: Maximizes volume relative to material used. A sphere holds the most water for the least clay, but a slightly flattened sphere (oblate) is more stable.
• Flat or ringed base: Prevents tipping. A flat base or a wide foot ring provides stability. Round-bottomed vessels (like amphorae) require a stand or are designed to sit in sand.
• Thick walls: Water storage vessels should be 8-12 mm thick — thicker than tableware. This adds weight but improves durability and reduces the seepage rate through porous earthenware.
• Handles or lugs: Essential for vessels over 5 liters. Water is heavy (1 kg per liter). A 20-liter vessel weighs 20 kg when full plus the weight of the clay body. Handles must be solidly attached — scored, slipped, and reinforced with coils at the attachment points.

Size Recommendations

Use	Volume	Diameter	Height	Notes
Personal drinking	1-2 liters	12-15 cm	15-20 cm	Portable, one-hand carry
Household daily use	5-10 liters	20-25 cm	30-35 cm	Beside cooking area
Storage cistern	20-50 liters	30-45 cm	40-60 cm	Stationary; fill with smaller vessels
Fermentation crock	10-30 liters	25-35 cm	35-45 cm	Wide mouth for access; needs lid

Making Watertight Pottery

Method 1: High-Temperature Firing (Best)

The most reliable waterproofing is vitrification — firing clay hot enough that glass phases form between the clay particles, sealing the pores naturally.

• Stoneware clays vitrify at 1,200-1,300C
• Some earthenware clays partially vitrify at 1,050-1,150C
• Porcelain clays fully vitrify at 1,250-1,350C

A fully vitrified vessel is naturally waterproof without any additional treatment. If your kiln reaches these temperatures and your clay body supports it, this is the ideal approach.

Testing vitrification: After firing, weigh the dry pot. Submerge it in water for 24 hours. Weigh it again. If it gained less than 1% of its dry weight in water, it is effectively vitrified and waterproof.

Method 2: Glazing (Most Common)

Applying a glassy glaze to the interior surface seals the pores completely. Even porous, low-fired earthenware becomes waterproof when properly glazed on the inside.

1. Bisque fire the vessel first (fire without glaze to approximately 800-900C)
2. Apply glaze to the interior by pouring:
   • Fill the vessel with glaze slurry (mixed to heavy cream consistency)
   • Swirl to coat all interior surfaces
   • Pour out the excess
   • Let drip for 10 seconds, then rotate to distribute evenly
3. Wipe glaze from the rim if you are not glazing the exterior (prevents sticking to kiln furniture)
4. Fire to the glaze’s maturation temperature

Interior-Only Glazing

For water vessels, you only need to glaze the interior. Leaving the exterior unglazed has an advantage: the slight porosity allows evaporative cooling. Water seeps microscopically through the wall, evaporates on the outside surface, and cools the remaining water inside. This is the principle behind traditional ceramic water coolers used across hot climates.

Method 3: Burnishing (No Glaze Required)

Burnishing compresses the surface clay particles into a dense, smooth layer that dramatically reduces porosity. It does not make the vessel perfectly waterproof but reduces seepage by 80-90%.

1. Wait until the vessel is leather-hard (firm but still slightly damp)
2. Rub the surface firmly with a smooth, hard object: a polished stone, the back of a spoon, a glass bottle, or a piece of bone
3. Work in overlapping strokes, pressing hard enough to create a slight sheen
4. Cover the entire surface — inside and out
5. Fire to no higher than 900C. Higher temperatures destroy the burnished surface as the clay reorganizes.

Burnished vessels have a beautiful satin finish and are functional for daily water use. They seep slowly enough for water storage of 1-3 days. For longer storage, combine burnishing with one of the sealant methods below.

Method 4: Interior Sealants (For Low-Fire Pottery)

When you cannot achieve high temperatures or produce glaze, you can seal the interior with natural waterproofing materials applied after firing.

Sealant	Application	Waterproofing	Food Safety	Durability
Beeswax	Melt and pour into warm pot, swirl to coat	Excellent	Safe	Months; re-apply when scratched
Pine resin / pitch	Heat and paint on interior	Excellent	Safe (used for millennia)	Years
Milk protein (casein)	Fill with milk, let sour, empty, dry, repeat 3x	Good	Safe	Months
Boiled linseed oil	Coat interior, bake at 150C for 2 hours	Good	Safe once cured	Years
Rendered fat / tallow	Rub into warm pot	Moderate	Safe	Weeks; goes rancid

Beeswax method in detail:

1. Fire the vessel and let it cool
2. Preheat the vessel by placing it near (not in) a fire until warm to the touch
3. Melt beeswax in a separate container (melts at 62-64C)
4. Pour molten wax into the warm vessel
5. Immediately tilt and rotate to coat all interior surfaces
6. Pour out excess wax
7. The residual heat keeps the wax liquid long enough to create a thin, even coating
8. Let cool completely before filling with water

Heat Limitation

Wax and resin sealants melt at moderate temperatures. Beeswax-sealed vessels must not be used for boiling water or placed near fire. They are for cold water storage only. For cooking vessels, use glaze or high-temperature firing instead.

Lids and Covers

An open water vessel loses water to evaporation and gains contamination. Every storage vessel needs a cover.

Fitted Clay Lid

1. Throw or slab-build a disc slightly larger than the vessel mouth
2. Add a rim flange on the underside that fits inside the vessel opening (prevents the lid from sliding off)
3. Add a knob or handle on top for lifting
4. Fire the lid with the vessel if possible (they shrink at the same rate and fit perfectly)

Water-Seal Lid (Fermentation Airlock)

For fermentation crocks, a water-seal lid prevents air from entering while allowing fermentation gases to escape:

1. Form a trough (channel) around the rim of the vessel — 2-3 cm wide, 3-4 cm deep
2. Make a lid with a downward-facing flange that sits in the trough
3. Fill the trough with water
4. The lid’s flange submerges in the water, creating an airtight seal
5. Gas pressure pushes bubbles out through the water; air cannot push back in

Evaporative Cooling Vessels

In hot climates, deliberately porous water vessels keep drinking water cool without ice or refrigeration.

How It Works

Water seeps through the porous clay wall at a very slow rate. On the outer surface, it evaporates. Evaporation absorbs heat energy from the remaining water inside the vessel. In dry climates with low humidity, this effect can cool water 10-15C below ambient temperature.

Design for Cooling

• Use unglazed, lightly fired earthenware (850-950C)
• Do not burnish the exterior — you want porosity
• Wall thickness 8-10 mm (thinner walls cool more but seep faster)
• Place in a shaded, breezy location for maximum evaporation
• Keep the vessel full — a half-empty vessel cools less effectively

The Two-Pot System

A traditional design from South Asia and Africa:

1. Place a small, porous pot inside a larger pot
2. Fill the gap between them with wet sand
3. Fill the inner pot with drinking water
4. Cover the top
5. Keep the sand wet

The sand’s evaporation cools the inner pot. The inner pot’s own wall porosity adds additional cooling. Combined effect: 15-20C below ambient temperature. Food stored inside stays fresh 3-5 times longer than at room temperature.

Testing Water Vessels

Before putting a water vessel into service, test it:

1. Fill test: Fill with water to the brim. Mark the water level. Wait 24 hours. Measure any drop.

   • Less than 2 mm drop: excellent, suitable for long-term storage
   • 2-5 mm drop: acceptable for daily use, refill regularly
   • More than 5 mm drop: needs additional sealing treatment

2. Leak test: Fill with water and place on a dry surface. After 1 hour, check for wet spots on the exterior or puddles underneath. Any external wetness indicates a crack or severe porosity.

3. Taste test: Fill with water, let sit for 24 hours, then taste. The water should taste clean. An earthy or chemical taste indicates the clay body or glaze is leaching minerals. Re-fire at a higher temperature or apply a different sealant.

4. Strength test: Rap the side of the vessel with your knuckle. A clear, ringing tone indicates a well-fired, dense body. A dull thud indicates under-firing or cracks.

Water vessels are the single most important product a potter makes for a rebuilding community. Get them right, and you solve water storage. Get them wrong, and water remains a daily crisis of collection, contamination, and waste.