How Do They Make Water Hourglass Glass? Step-by-Step Process

The short answer is that water hourglass glass is made using traditional glassblowing techniques combined with careful sealing and calibration. The process is not automated; it relies on skilled glassblowers who shape the glass, create the internal constriction, and seal the water inside.

Raw Materials and Initial Preparation

The process starts with a glass tube, usually made from borosilicate glass. This type of glass resists thermal shock and has a low coefficient of expansion, meaning it will not crack easily when the hot glass is worked or when the finished piece is handled. The tube is typically clear and has a consistent wall thickness.

The glassblower cuts the tube to a length that will become the main body of the hourglass. The ends are then fire-polished to remove sharp edges and prevent stress points.

Shaping the Glass and Creating the Constriction

The glassblower uses a bench-mounted torch, often a propane-oxygen or natural gas-oxygen mix, to heat a specific section of the tube. The heat softens the glass to a workable state, typically around 900 to 1100 degrees Celsius for borosilicate glass. The glassblower rotates the tube continuously to ensure even heating.

Once the glass is soft, the blower gently pulls the two ends of the tube apart. This creates a narrow waist, or constriction, in the center. The constriction is the key feature of a water hourglass. It must be narrow enough to slow the flow of water but wide enough to allow air bubbles to pass through as the water moves from one chamber to the other.

The blower uses tools like graphite paddles or wet newspaper to shape the constriction. The goal is to create a smooth, symmetrical opening, usually between 2 and 5 millimeters in diameter. If the opening is too small, the water will not flow. If it is too large, the hourglass will run too quickly. The blower often makes several passes, reheating and adjusting the shape until it is correct.

After the constriction is formed, the glassblower heats the ends of the tube and blows air into them to create the upper and lower bulbs. These bulbs hold the water. The blower controls the size of each bulb by how much air is blown and how the glass is shaped.

Sealing the Water Inside

This is the most delicate part of the process. The glassblower must trap a precise amount of water inside the hourglass without any air bubbles or contaminants.

The water used is typically distilled water. It is often mixed with a small amount of alcohol or a surfactant to reduce surface tension. This helps the water flow smoothly through the constriction without sticking or forming droplets. Some makers add a few drops of food coloring to make the water visible. The water is filtered to remove any particles that could clog the constriction.

The glassblower attaches a small funnel or a thin tube to one end of the glass piece. The water is poured in through this opening. The amount of water is critical. Too much water will leave no air space, and the hourglass will not function. Too little water will cause the hourglass to run too quickly or not at all.

Once the correct amount of water is added, the glassblower heats the open end of the tube. The heat causes the water to boil slightly, which pushes out excess air. The glassblower then seals the end by melting the glass shut. This creates a vacuum inside the hourglass. The vacuum is important because it allows the water to flow freely without being slowed by air pressure differences.

Annealing

The sealed end is then annealed. Annealing is a slow cooling process that relieves internal stresses in the glass. The hourglass is placed in a kiln and cooled gradually over several hours. If the glass is cooled too quickly, it can crack or shatter. A properly annealed hourglass can last for decades without cracking.

Calibrating the Flow

After the hourglass is sealed and annealed, it must be tested and calibrated. The glassblower turns the hourglass over and times how long it takes for the water to flow from the upper bulb to the lower bulb. If the time is too short or too long, the glassblower may need to adjust the constriction.

Adjusting the constriction after the glass is sealed is difficult. The glassblower may need to reheat the constriction area and gently squeeze or expand it with tools. This is a risky step because the glass can break. Some makers instead adjust the water volume by adding or removing a small amount of water through a tiny hole, which is then resealed.

The calibration process is repeated until the hourglass runs for the desired time. A common desktop water hourglass might run for 5 to 15 minutes, but custom pieces can run for hours.

Key Takeaways

• Water hourglass glass is made from borosilicate glass using a bench torch and traditional glassblowing techniques.
• The constriction is the critical feature; it is formed by heating and pulling the glass tube.
• Distilled water, often mixed with alcohol or dye, is sealed inside under a partial vacuum.
• Calibration involves timing the flow and adjusting the constriction or water volume.
• The process requires skill and precision; each piece is handmade and unique.