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Jeff the transparent blue swirls are Ravenswood Novelty. 

The Vitro piece is fire brick not stone. It is not a mold. It is a crucible, this is where the colors were added to the base glass.  It is a piece of the equipment or furnace used in producing the Vitro six vane Cage Cat Eye marbles. It is placed or hangs inside near the front of the furnace, near the outlet or exit hole. The base glass flows in and around the six partitions and comes out around the bottom. The colored glass is feed into the smaller six holes which exit the bottom in a circle. The base glass flows around the outside and between the colors as all of them reach the bottom. The clear base encases the smaller colored glass streams.  The marble ends up with the majority a clear glass center and clear glass around the outside, with the smaller streams of colored glass swirling in the clear glass. When a hole plugs then you have five vane cage cat eyes. Crucibles do not last long and have to be replaced often. Big temperature changes inside the furnace affect the life of the crucible. The fire brick of the crucible is thin compared to the furnace walls. The fire brick for most marble furnace walls is about four to six inches thick. The floor or bottom furnace brick of most marble furnaces are about twelve to twenty inches thick. Bottom furnace bricks can be 16 inches wide, 24 inches long and 12 inches thick. Sometimes the entire bottom of a crucible may burn out and fall off completely in the furnace. It is then removed if possible or moved to the side. If possible a new crucible is installed on the fly while running. So some furnace brick pieces will end up in some marbles. Most of those will be discarded. But as always some will escape to collectors. There are a big variety of crucibles, diameter, length, number of holes, Some have only one hole. More than one crucible can be installed in a furnace.  I have seen two but not three in a furnace. This is just one of many methods of adding color to the base glass. Machine made glass marble patterns, style or designs all happen upstream before the actual marble machine. The base glass meets the smaller color glass stream or streams. It all flows and twist as gravity pulls it from the furnace to the shear. Clockwise like water down a drain. So the colored lines in the base glass might twist depending on long the stream is to the shear.  Short stream equals more straight lines. A longer stream equals more twist.  But if to long the glass will get to cold and cause major problems. Machine made marble making is finding all the sweet spots and keeping things there. But they change often. During the investor runs at Jabo, we had two Jabo operators for one machine, and lots of helpers. Many times every person was busy trying to get things back in order.  Every minute during a problem, glass is being lost. About 250 marbles per minute and each investor wanted every marble they could get.  The crucible above was dug at Vitro Agate Parkersburg WV.  The pictures are of one single broken crucible.  A new crucible would be much longer than this piece. This is a bottom which has broken off after lots of use.  A new one is maybe 16 or 24 inches long or more, can be four or six inches diameter and like a hallow tube or pipe shape.  They can vary for different furnace configurations, sizes and uses.  

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