ABSTRACT

Smalt is a blue cobalt-coloured glass pigment used in European wall paintings from the early Renaissance period that became a popular choice for artists due to its unique tone, availability, and versatility. The pigment has long been considered as stable in an alkaline environment since it was applied with lime-based techniques and generally retained its colour over time. This perceived stability has meant that aqueous-based alkaline treatments, often involving high pH values, substantial quantities of liquid, and long contact times, have been used in conservation treatments. However, studies of the kinetics of glass deterioration in the past few years have demonstrated that glass is highly reactive in contact with water and substances with both high and low pH values. Research into smalt deterioration in canvas paintings has shown that smalt is susceptible to elemental leaching and physical changes in contact with water and low pH values; however, there is little specific information about the conservation implications of these findings on the treatment of smalt in wall paintings. This experimental study explores several aspects of how aqueous-based alkaline conservation treatments affect smalt grains in lime-based wall paintings: the influence of painting technique, exposure of the pictorial layer to water before treatment, and pH values and contact times during treatment. Replicas representing three different painting techniques were created, exposed to liquid water or water vapour, and treated with three substances commonly used in conservation according to accepted practices in terms of preparation and application. Chemical and physical changes in the pigment were then analysed using scanning electron microscopy with energy dispersive X-ray spectroscopy. Results confirmed that aqueous-based alkaline substances provoke chemical changes as well as physical deterioration in smalt applied in lime-based techniques evidenced by leaching, corrosion, and cracking in the pigment grains. Crucial factors found to influence the impact of treatments on smalt included the previous exposure to humidity of the pictorial layer, the pH value of the treatment, and extended contact times.

摘要

Smalt 是一种钴蓝色玻璃颜料，用于文艺复兴早期的欧洲壁画，因其独特的色调、可用性和多功能性而成为艺术家的热门选择。长期以来，这种颜料一直被认为在碱性环境中是稳定的，因为它是用基于石灰的技术应用的，并且通常会随着时间的推移保持其颜色。这种可感知的稳定性意味着水基碱性处理（通常涉及高 pH 值、大量液体和长时间接触）已用于保护处理。然而，过去几年对玻璃劣化动力学的研究表明，玻璃在与水和具有高 pH 值和低 pH 值的物质接触时具有很强的反应性。对帆布画中麦芽变质的研究表明，麦芽容易受到元素浸出和与水接触的物理变化以及低 pH 值的影响；然而，关于这些发现对壁画中 smalt 处理的保护意义的具体信息很少。本实验研究探讨了水性碱性保护处理如何影响石灰壁画中的细粒：绘画技术的影响、处理前图案层与水的接触，以及处理期间的 pH 值和接触时间。代表三种不同绘画技术的复制品被创造出来，暴露在液态水或水蒸气中，并根据制备和应用方面的公认惯例，用三种常用于保护的物质进行处理。然后使用具有能量色散 X 射线光谱的扫描电子显微镜分析颜料的化学和物理变化。结果证实，水基碱性物质会引起化学变化以及石灰基技术中应用的 smalt 的物理劣化，表现为颜料颗粒的浸出、腐蚀和开裂。发现影响处理对 smalt 影响的关键因素包括先前暴露于图案层的湿度、处理的 pH 值和延长的接触时间。结果证实，水基碱性物质会引起化学变化以及石灰基技术中应用的 smalt 的物理劣化，表现为颜料颗粒的浸出、腐蚀和开裂。发现影响处理对 smalt 影响的关键因素包括先前暴露于图案层的湿度、处理的 pH 值和延长的接触时间。结果证实，水基碱性物质会引起化学变化以及石灰基技术中应用的 smalt 的物理劣化，表现为颜料颗粒的浸出、腐蚀和开裂。发现影响处理对 smalt 影响的关键因素包括先前暴露于图案层的湿度、处理的 pH 值和延长的接触时间。