Only a century ago, ice was hard to come by in most parts of the world. In hotter climates, you had to buy your ice from a delivery service, which imported hefty blocks from a colder climate or from an industrial refrigeration plant. The price of ice was relatively steep, but if you wanted to keep your food cold, you didn't have much choice. In the hottest parts of the world, ice was a rare luxury. In an equatorial country, you might live your whole life and never even see a piece of ice.

This all changed in the early 20th century. Compact, affordable refrigerators brought the means of food preservation and ice production into the home and corner store. In the 1960s, new automatic icemaker machines made life even easier. These days, most Americans take ice completely for granted, even during the hottest days of summer.

In this article, we'll find out what's inside a typical home icemaker, as well as the larger commercial icemakers you might find at a hotel or grocery store. As we'll see, the basic process of making ice is very simple -- you just freeze water -- but spitting out perfectly shaped ice cubes is a fairly elaborate process.

The home icemaker's predecessor was the plastic ice tray. It's fairly obvious how this device works: You pour water into a mold, leave it in the freezer until it turns to a solid and then extract the ice cubes. An icemaker does exactly the same thing, but the process of pouring water and extracting cubes is fully automated. A home icemaker is an ice-cube assembly line.

The home icemaker is a miniature ice-cube assembly line.

The home icemaker is a miniature ice-cube assembly line.

Most icemakers use an electric motor, an electrically operated water valve and an electrical heating unit. To provide power to all these elements, you have to hook the icemaker up to the electrical circuit powering your refrigerator. You also have to hook the icemaker up to the plumbing line in your house, to provide fresh water for the ice cubes. The power line and the water-intake tube both run through a hole in the back of the freezer.

In the next section, we'll look at the cycle an icemaker goes through to make ice.

The icemaker has a heating coil underneath the ice mold.

The icemaker has a heating coil underneath the ice mold.

When everything is hooked up, the icemaker begins its cycle. The cycle is usually controlled by a simple electrical circuit and a series of switches. In the diagram below, you can see how the icemaker moves through its cycle.

At the beginning of the cycle, a timed switch in the circuit briefly sends current to a solenoid water valve. In most designs, the water valve is actually positioned behind the refrigerator, but it is connected to the central circuit via electrical wires. When the circuit sends current down these wires, the charge moves a solenoid (a type of electromagnet), which opens the valve.

The valve is only open for about seven seconds; it lets in just enough water to fill the ice mold. The ice mold is a plastic well, with several connected cavities. Typically, these cavities have a curved, half-circle shape. Each of the cavity walls has a small notch in it so each ice cube will be attached to the cube next to it.

Once the mold is filled, the machine waits for the water in the mold to freeze. The cooling unit in the refrigerator does the actual work of freezing the water, not the icemaker itself (see How Refrigerators Work for details). The icemaker has a built-in thermostat, which monitors the temperature level of the water in the molds. When the temperature dips to a particular level -- say, 9 degrees Fahrenheit (-13 degrees Celsius) -- the thermostat closes a switch in the electrical circuit (see How Home Thermostats Work for details on this operation).

Closing this switch lets electrical current flow through a heating coil underneath the icemaker. As the coil heats up, it warms the bottom of the ice mold, loosening the ice cubes from the mold surface.

The electrical circuit then activates the icemaker's motor. The motor spins a gear, which rotates another gear attached to a long plastic shaft. The shaft has a series of ejector blades extending out from it. As the blades revolve, they scoop the ice cubes up and out of the mold, pushing them to the front of the icemaker. Since the cubes are connected to one another, they move as a single unit.

At the front of the flake ice machine, there are plastic notches in the housing that match up with the ejector blades. The blades pass through these notches, and the cubes are pushed out to a collection bin underneath the icemaker.

The revolving shaft has a notched plastic cam at its base. Just before the cubes are pushed out of the icemaker, the cam catches hold of the shut-off arm, lifting it up. After the cubes are ejected, the arm falls down again. When the arm reaches its lowest resting position, it throws a switch in the circuit, which activates the water valve to begin another cycle. If the arm can't reach its lowest position, because there are stacked-up ice cubes in the way, the cycle is interrupted. This keeps the icemaker from filling your entire freezer with ice; it will only make more cubes when there is room in the collection bin.

This system is effective for making ice at home, but it doesn't produce enough ice for commercial purposes, such as restaurants and self-service hotel ice machines. In the next section, we'll look at a larger, more powerful icemaker design.

In this system, the metal ice tray is connected to a set of coiled heat-exchanging pipes like the ones on the back of your refrigerator. If you've read How Refrigerators Work, then you know how these pipes work. A compressor drives a stream of refrigerant fluid in a continuous cycle of condensation and expansion. Basically, the compressor forces refrigerant through a narrow tube (called the condenser) to condense it, and then releases it into a wider tube (called the evaporator), where it can expand.

Compressing the refrigerant raises its pressure, which increases its temperature. As the refrigerant passes through the narrow condenser coils, it loses heat to the cooler air outside, and it condenses into a liquid. When the compressed fluid passes through the expansion valve, it evaporates -- it expands to become a gas. This evaporation process draws in heat energy from the metal pipes and the air around the refrigerant. This cools the pipes and the attached metal ice tray.

The icemaker has a water pump, which draws water from a collection sump and pours it over the chilled ice tray. As the water flows over the tray, it gradually freezes, building up ice cubes in the well of the tray. When you freeze water layer by layer this way, it forms clear ice. When you freeze it all at once, as in the home icemaker, you get cloudy ice (see How do you make clear ice cubes? for more information).

After a set amount of time, the icemaker triggers a solenoid valve connected to the heat-exchanging coils. Switching this valve changes the path of the refrigerant. The compressor stops forcing the heated gas from the compressor into the narrow condenser; instead, it forces the gas into a wide bypass tube. The hot gas is cycled back to the evaporator without condensing. When you force this hot gas through the evaporator pipes, the pipes and the ice tray heat up rapidly, which loosens the ice cubes.

Typically, the individual cube cavities are slanted so the loosened ice will slide out on their own, into a collection bin below. Some systems have a cylinder piston that gives the tray a little shove, knocking the cubes loose.

This sort of system is popular in restaurants and hotels because it makes ice cubes with a standard shape and size. Other businesses, such as grocery stores and scientific research firms, need smaller ice flakes for packing perishable items. We'll look at flake icemakers next.