On modern (post 1930's) spark plugs, the tip of the insulator protruding into the combustion chamber is the same sintered aluminium oxide (alumina) ceramic as the upper portion, merely unglazed. It is designed to withstand 650 °C (1,200 °F) and 60,000 volts.
The dimensions of the insulator and the metal conductor core determine the heat range of the plug. Short insulators are usually "cooler" plugs, while "hotter" plugs are made with a lengthened path to the metal body, though this also depends on the thermally conductive metal core.
Older spark plugs, particularly in aircraft, used an insulator made of stacked layers of mica, compressed by tension in the centre electrode.

Illustration of compressed mica insulator from 1928.


With the development of leaded petrol in the 1930s, lead deposits on the mica became a problem and reduced the interval between needing to clean the spark plug. Sintered alumina was developed by Siemens in Germany to counteract this.[6] Sintered alumina is a superior material to mica or porcelain because it is a relatively good thermal conductor for a ceramic, it maintains good mechanical strength and (thermal) shock resistance at higher temperatures, and this ability to run hot allows it to be run at "self cleaning" temperatures without rapid degradation. It also allows a simple single piece construction at low cost but high mechanical reliability.

At one time it was common to remove the spark plugs, clean deposits off the ends either manually or with specialized sandblasting equipment and file the end of the electrode to restore the sharp edges, but this practice has become less frequent for two reasons: 1. cleaning with tools such as a wire brush leaves traces of metal on the insulator which can provide a weak conduction path and thus weaken the spark (increasing emissions) 2. plugs are so cheap relative to labor cost, economics dictate replacement, particularly with modern long-life plugs.

The development of noble metal high temperature electrodes (using metals such as yttrium, iridium, tungsten, or palladium, as well as the relatively high value platinum, silver or gold) allows the use of a smaller center wire, which has sharper edges but will not melt or corrode away. These materials are used because of their high melting points and durability, not because of their electrical conductivity (which is irrelevant in series with the plug resistor or wires). The smaller electrode also absorbs less heat from the spark and initial flame energy. At one point, Firestone marketed plugs with polonium in the tip, under the (questionable) theory that the radioactivity would ionize the air in the gap, easing spark formation.

The newest spark plug technology can be summed up in one word: Iridium. This relatively rare metal is challenging platinum as the metal of choice for today’s spark plugs. Platinum has held that title since 1985, when Bosch introduced its first platinum spark plugs. The heat- and wear-resistant properties of platinum electrodes allows most of these plugs to go upward of 100,000 miles before they have to be replaced.

For automakers and motorists who want longer spark plug service intervals, platinum plugs have been an excellent upgrade over standard plugs. Platinum plugs do command a higher price, sometimes two to four times more than a standard plug. Even so, when the much longer service life of platinum plugs is taken into account, they are usually more economical than standard spark plugs.

When platinum plugs first appeared, they were used primarily in a handful of European luxury makes. As the list of OEM applications grew, other vehicle manufacturers began clamoring for long-life plugs, too. Before long, other spark plug suppliers began offering their own long-life plugs, including some with gold palladium electrodes. Most, however, offered either single- or double-platinum electrodes (the latter having a small platinum button welded to both the center and ground electrodes).

Long-life platinum plugs also created a whole new segment for premium replacement spark plugs in the aftermarket. Prior to this, parts stores could only offer standard replacement spark plugs (either the OEM brand or a competing brand), or some type of “performance” spark plug with an enhanced electrode design that reduced misfires and improved performance. A lot of “hocus pocus” science and marketing hype surrounded many of these electrode designs. Yet in spite of the claims that were being made for some of these plugs, none could match the longevity of platinum plugs.

IRIDIUM: THE CHALLENGER
In 1994, the first iridium spark plugs were introduced by NGK as a long-life alternative to platinum plugs. Why iridium? Because iridium has a higher melting temperature than platinum, it is six times harder than platinum, and it is more corrosion-resistant than platinum or most other metals. NGK developed a fine wire electrode with an iridium tip that offered many of the advantages of a performance spark plug (concentrated spark and reduced firing voltage requirements) with long wear characteristics comparable to or possibly even better than platinum.

As with platinum spark plugs, the initial use of iridium plugs was limited to a handful of import makes (Japanese nameplates). But as the list of OEM applications has grown, so has the demand for aftermarket iridium replacement spark plugs. Most spark plug suppliers want the ability to offer their customers replacement spark plugs that are OEM equivalent in their product lines. If they don’t have iridium spark plugs, chances are they will lose sales to competitors who do. And as more domestic and European automakers switch to iridium plugs, the demand will continue to grow.