Also found this information on the forum from a post submitted back on July 10th. I am assuming from the writing style that this was written by Walter Clifford as well:

"Cerec refers to a system of CAD equipment from the Sirona Company which carves crowns and the like from pre-formed blocks of ceramic and porcelain. Other companies are coming into the marketplace with similar equipment. We currently have in our system some 32 different kinds of porcelain/ceramic blocks for use with the Cerec-type machines. Some of the blocks are aluminosilicate (aluminum salt) based and others are zircate (zirconium salt) based. Aluminum and zirconium are both metals. I would need to speak with the doctor to determine which of the 32 types of blocks is proposed for use before I could comment on potential suitability.

In spite of all the hoopla and advertising signs put out by dentists, there is no such animal as a metal-free restorative. >From basic freshman chemistry class, whenever there is an anion in nature, there must be a balancing cation. Cations are nearly always metals. Hence, there is metal even in simple denture acrylic. All of the porcelains, ceramics, glass ionomers, composites, compomers, ceromers and the like contain metals. At a minimum, these may include metals such as barium, lithium, iron, titanium, cobalt, aluminum, zirconium and manganese. The important question to ask is not whether or not there are metals, but rather, what forms do the metals take.

The more severe galvanic problems and concomitant release of ionized metals take place with frankly reduced (shiny, metallic-looking) metals. The oxidized forms of the metals (metal salts) are often less problematic. Porcelains and ceramics usually have oxidized metals in a crystalline lattice formation, and these are more stable still. Any form of metal can be problematic if there are sufficient differences in the bio-materials present in the body. I don’t have time to provide a complete course in chemistry and physics on the matter, but the secret to minimizing the off-loading of whatever metals may be present in a bio-available form is to keep all restorative products within the same general chemical groups and families so that electromotive forces are minimal.

As to aluminum, we get really excited about certain forms of aluminum. If the only forms of aluminum present are aluminum oxide, alumina or aluminosilicate, we will totally ignore the aluminum unless the patient also has a silicate sensitivity. To become toxic and bio-available, aluminum must be ionized. The forms of aluminum compounds mentioned above have a dissociation half-life between 109 and 1011 years under expected oral cavity conditions. In simple terms, there will not be enough dissociated aluminum from restoratives to measure in a lifetime. It is true that you can physically break off micro-fines of the restorative, but these are small bits of intact compound and not dissociated aluminum. They will void in the next bowel movement or two. To put this into proper perspective, your skeleton is composed of 2.0% – 2.5% aluminum oxide and aluminosilicate, normally and naturally. There is more aluminosilicate in a single side serving of fresh garden salad containing lettuces and vegetables than will be released from a mouth full of ceramic or porcelain crowns in a normal lifetime. The plants pick up the aluminosilicates from the soil readily and incorporate them into their tissues.

The bottom line is that there are some forms of aluminum compounds which are a non-issue, and this is seen in some of the Cerec blocks. The zircate-based blocks may be a different matter, depending upon the individual patient. "