Monolithic and stained individualized zirconia restorations offer attractive aesthetics and allow for a substance-saving preparation. By choosing a suitable attachment material, acceptable results can be achieved even on stained stumps. However, experts often disagree when it comes to the color assessment of zirconium restorations. Especially since the zircons have become more translucent, many factors influence the color effect. In addition to the degree of translucency for example, the material thickness of the construction, the firing program used, sintering conditions, and the type of glaze are some other factors (Fig. 1). Most relevant, however, is the material’s own color chroma of the zirconia.

Zirkonoxid und Farbtreue. Ein ungleiches Paar? Blogbeitrag

Fig.1: Material: DD cubeX² - 5Y TZP (Dental Direkt GmbH) – pre-dyed in A2. Right after the sintering. Center only glazed. Left glazed and colored. The zirconium color should be optimized for the desired result after glaze. The glaze creates a different light refraction and thus a changed color value.

So how does color develop in zirconia and what are the advantages and disadvantages of the available dyeing systems?

Basically, there are two main dyeing systems. On the one hand, it is possible to introduce appropriate coloring ions into the porous structure of the milled construction in the white state by using dyeing solutions. On the other hand, milling from pre-colored blanks is possible. The effect of color development is similar in both systems.

In order to achieve a color effect, oxides or ions from rare earth or transition metals are usually used. During the sintering process, these are mostly deposited as oxides at the grain boundaries of the zirconia, partly integrated into the crystal lattice of the material (see Fig. 2).

Zirkonoxid und Farbtreue. Ein ungleichers PAar? abb. 2 Blogbeitrag

Fig. 2: During the sintering process, the coloring oxides are deposited at the grain boundaries and partially integrated into the crystal lattice. The color effect finally arises by reflection and absorption of certain wavelengths of the incident light.

The incident light interacts with the final sintered structure. This results in the reflection of the light and absorption of certain wavelengths, whereby the desired color effect arises.
The use of staining solutions offers the possibility of influencing the individual color design of the denture in the white state, before the final sintering, but in practical application may have more variables than the use of pre-stained materials. 

Finally, the color obtained depends on the amount and distribution of the coloring oxides or ions in the material. For example, constructions of different wall thicknesses require different amounts of liquid to be infiltrated. Especially when using what’s known as the painting technique, it takes a lot of experience to achieve consistent color results in the very different constructions in everyday dental technology. A different saturation of the construction with coloring ingredients has a significant effect on the resulting chroma. For example, massive structures often have a more intense color than thin copings. Not least, this staining technique is limited by the maximum achievable depth of infiltration of the staining solutions from the outset. In addition, it is necessary to dry the structures infiltrated with liquid before sintering, which means a longer processing time. If wet milled or sanded before dyeing, it is advisable to burn out the coolant additives, as otherwise irregular (blotchy) color results may occur.  
The reproducibility and reliability of color results, however, is crucial for success in everyday dental technology.

By using raw materials of high purity and optimized industrial manufacturing processes, pre-dyed zirconia materials ensure a homogeneous distribution of the coloring oxides or ions in the entire structure of the dental restorations. A high manufacturing consistency in the industrial processes leads to the greatest possible safety and reproducibility of the color results, regardless of the design of the constructions. The time required for liquid dyeing and drying can be omitted. 
The corresponding individualization of the denture is finally possible with both techniques by targeted application of a reduced veneer (cut back) or the use of glaze and stains.

Color measurement and evaluation

When selecting and communicating tooth colors, it is customary to use the color schemes established in dentistry and dental technology, such as the VITA® color system. This divides the colors into four different color groups with shades of various intensity. For example, the A-series is defined as reddish-brownish.
An established method for measuring, describing, and evaluating precisely these colors in industrial production is the L*a*b* System. It describes a three-dimensional color space in which with the brightness value L* and the color coordinates a* (+a red/a green) and b* (+b* yellow/b* blue), all colors in tone and chroma/saturation by numerical values can be displayed (see Fig.3).

Zirkonoxid und Farbterue. Ein ungleiches Paar? Abb.3 Blogbeitrag

Fig. 3: Image source Konica Minolta Fig. Slightly modified.

In turn, color differences can be described by delta values. From a certain delta value, such a measurable color difference is no longer perceptible to the human eye. Nevertheless, it is indispensable for evaluating colors also visually on the basis of dental engineering constructions, since ultimately shape, wall thickness, and surface quality of the constructions lead to different color effects. It should also consider the environment, as well as the angle and the type of incident light and ideally  
be varied. Likewise, the evaluation of tooth colors on the basis of constructions in the sintered and finally glazed state absolutely makes sense, since glaze masses can change the color result. 

The choice of the user as to which is the better procedure in-house is therefore very individual. Factors such as the operators’ expectations with regard to color consistency, economic aspects of storage inventory, and production times should be balanced against one another.