The acids applied in this study are commonly
The acids applied in this study are commonly used in dental practice. Phosphoric Anacardic acid is an oxyacid used regularly for adhesion of resin composites. Enamel phosphoric acid etching can dissolve and demineralize the inorganic [Ca10(PO4)6(OH)2] matrix, creating micro pores and microgrooves to improve mechanical retention, and increases the attraction of positively charged molecules of dyes [27,28]. Oppositely, the hydrofluoric acid is a hydracid used previously to cementation of ceramics and it is known for dissolving glass-based structures . Therefore, the substrate for each acid is different, which explains the preference for enamel and filling colouring, respectively. Interestingly, the differences in particle sizes among brands had no influence in the coloring potential of the solutions.
It is important to emphasize that the hydrofluoric acid is known for its toxicity, which severity and symptoms depend on its concentration, the duration of exposure to it, and the penetration in the tissues . To the best of the authors´ knowledge, no incident related to its use in dentistry has been reported and, therefore, the 5% concentration seems to be safe . Nevertheless, it is recommended that the operator wears personal protective equipment, especially gloves, at all times. Anyway, phosphoric acid would be the preferred method concerning occupational health risks and for the clinics.
With respect to the duration of acid-etching, few studies have shown that ceramic etching with a 5% HF during for 2–3 min is sufficient to dissolve the glassy phase , increasing roughness and creating a surface more favourable to bonding. Similarly, a previous study  showed that 10% hydrofluoric acid etching for 1 min produces a rougher surface in ceramics similar to enamel. In the early 1980’s phosphoric etching was set at 60 s, to obtain the preferred morphologic alteration of enamel prisms [34,35]. Therefore, in order to simplify the protocols and avoid unnecessary health risks, the etching time was set at 60 s for both acids, as our aim was not to promote bonding, but only improve staining.
Conflict of interest
Source of funding This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior — Brasil (CAPES) — Financial Code 001.