## 2 Materials and Methods
### Fabrication of the Three Substrates  
#### 2.1.1 Natural Tooth Substrate  
For the fabrication of the natural tooth substrate, a sound premolar tooth, previously extracted for orthodontic treatment reasons, was selected. The study protocol was reviewed and approved by the Ethics Committee for Life of the Faculty of Lisbon, Universidade de Lisboa. The donation of the tooth for research purposes was possible after the donor provided informed consent.  
The tooth was positioned within a transparent acrylic cube, leaving the coronal portion exposed, ensuring a stable and suitable support (Figure 1).  
To ensure a consistent positioning for color measurement of the coronal surface, a pink silicone key was created, allowing a VITA Easyshade(r) V (VITA Zahnfabrik, Bad Sackingen, Germany) spectrophotometer to be placed in the same position for each measurement. This  
Figure 1: Placement of the premolar tooth in a transparent acrylic block, ensuring proper support and exposure of the coronal area: (**a**) lateral view, (**b**) top view, (**c**) frontal view.  
ensured that all measurements were taken in the middle third of the buccal surface of the tooth (Figure 2).  
Three consecutive tooth color measurements were performed using the VITA Easyshade(r) V (VITA Zahnfabrik, Bad Sackingen, Germany) in the Natural Tooth Shade Determination program. The closest color obtained was A2, corresponding to the VITAPAN Classical shade guide (Figure 3).  
To preserve and reproduce the tooth shape, the acrylic block was placed in a S600 Arti Scanner (Zirkonzahn, Gais, Italy) and the premolar tooth was scanned using the Zirkonzahn Modellier software version 9530, generating the first STL file (STL1) (Figure 4).  
Figure 4: Digitization of the premolar tooth using the S6000 Arti scanner and the Zirkonzahn Modellier software.  
Figure 3: Determination of the tooth color (VITA A2) upon selection on the menu of the equipment and after three consecutive color measurements performed with the Easyshade V spectrophotometer.  
Figure 2: Creation of a pink silicone key and placement of the Easyshade V spectrophotometer to ensure all measurements were consistently taken in the middle third of the buccal surface of the tooth. (**a**) Position; (**b**) reading.  
Tooth preparation was guided using a silicone matrix to control a homothetic 1.5 mm thickness across all surfaces of the crown (Figure 5).  
After tooth preparation, a new scanning of the prepared tooth was performed, generating the second STL file (STL2).  
Considering the previously identified tooth shade (VITA A2), pre-shaded zirconia blocks from two commercial brands with different formulations were selected: Amann Girrbach (AG, Maeder, Austria)--Ceramill Zolid HT+ Preshades (98 mm in diameter, 14 mm in height, batch 190006); and Zirkonzahn (ZZ, Gais, Italy)--Anatomic Coloured zirconia (95 mm in diameter, 12 mm in height, batch ZB7173C).  
STL 1 and STL 2 files were processed in each brand's software, and 10 crowns were milled using a Ceramill Motion 2 milling machine (AG, Maeder, Austria); another 10 crowns were milled using an M1 Heavy Metal milling machine (ZZ, Gais, Italy) (Figure 6).  
The sintering cycles for the zirconia crowns were used according to the manufacturers' recommendations: for the AG brand, the crowns underwent a 10-h sintering process, reaching a maximum temperature of 1450 \({}^{\circ}\)C, and for the ZZ brand, the sintering cycle lasted 8 h, with a maximum temperature of 1500 \({}^{\circ}\)C.