The introduction of the Warlock model in 1981 helped push B.C. Rich into the heavy metal music genre. Notable early players included Mick Mars and Nikki Sixx of Mötley Crüe, Lita Ford, Paul Stanley of KISS (featuring a broken mirror top), Randy Jackson (Zebra), Craig Goldy of Dio and Giuffria, and a young pre-Les Paul playing Slash. Its popularity continues with players such as Slayer guitarist Kerry King. While Slipknot was gaining in popularity (early 2000's), guitarist Mick Thomson was well known for his signature red/black and black/white Warlocks with "HATE" fretboard inlays. A variation on the Warlock design, called the "Warbeast" has also been produced. The Warbeast features a more heavily bevelled body, longer body horns as well as an additional horn in the upper middle section. The Warbeast was also produced exclusively with the "3-in-line" headstock design, and with Floyd Rose bridges included as standard, except on the cheapest models. Designed by Joey Rico in 1983, the Ironbird gained some popularity amongst heavy metal guitarists, including Trey Azagthoth of Morbid Angel. The original model had a pointed reverse headstock, whereas the 21st Century version has a regular pointed headstock. These guitars are made completely of acrylic and their bodies are transparent, making the electronics inside visible. The original run of the acrylic models featured a standard bolt-on maple neck with wood headstock, but later models featured an acrylic headstock, matching the color of the body and making the overall appearance of the guitar more attractive. Acrylic is more dense than most woods (specific gravity of acrylic is 1.18 g/cm³ while that of lignum vitae, contender for the most dense type of wood, is approximately 1.23 g/cm³) which makes the guitar heavier. Like Rickenbacker and Jackson, B.C. Rich used a "neck-through" body design in many of their instruments. In 2006 they introduced the "IT" ("Invisibolt Technology") series, which combines elements of bolt-on and neck-through designs: The neck is bolted inside the body to make the guitar look much like a "neck-through", but the neck joint is still visible. B.C. Rich also pioneered the heel-less joint. Some models used custom battery-powered active electronics – pickups and tone controls inside the guitar.