Five or six thousand years ago, a ceramic craftsman accidentally discovered a shiny object in a pottery basin during the process of making pottery. Since then, glass has entered people’s field of vision. For thousands of years, glass has always been an indispensable part of people’s lives. In addition to its beautiful appearance, transparency, and texture, it is also closely related to its excellent qualities such as thermal insulation, electrical insulation, high hardness, and recyclability. Today, let us walk into the world of glass and get to know this special existence.
The composition and classification of glass
glass is composed of vitreous element, co-solvent, curing agent and other ingredients (including clarifier, coloring agent, decoloring agent and opacifying agent, etc.). Vitreous is mainly made of natural raw materials. Silica sand, sandstone, feldspar, kaolin, wax stone, limestone, etc. are all commonly used glass raw materials. According to different glass types, uses and technical requirements, the formulation design is quite different. Taking the soda lime glass for food contact as an example, the common formulations are shown in the following figure:
Soda lime glass
Glass products used for food contact can be divided into the following categories according to their chemical composition:
Glass products are based on chemical composition
The production process of glass seems simple, but there is a hidden mystery. Every link may have more or less impact on product quality and food safety:
Batching process: Generally, it includes three links: formula design, raw material pre-processing and batch preparation. Glass manufacturers usually consider the technical requirements, processing conditions, raw material acquisition and price of glass products when formulating. Different formulations will introduce different metal and non-metal elements. For example, silica sand and silica rock contain a small amount of Al2O3, Fe2O3, CaO and MgO in addition to the main component SiO2. Green glass may use the colorant cobalt oxide (CoO) And copper sulfate (CuSO4), etc., purple glass may use colorants such as manganese oxide (Mn203) and cobalt trioxide (Co2O3).
Glass melting is one of the important processes in glass production. It is a process in which raw and auxiliary materials are heated at high temperatures to prepare molten glass that can meet the molding requirements. The melting process is divided into five stages: silicate formation, glass formation, clarification, homogenization, and cooling. The first two stages usually require higher temperatures, usually 1200°C or higher. The control of the melting process directly affects product quality and product qualification rate.
The molding methods of glass products include centrifugal method, drawing method, pressing method, calendering method, blowing method, float method and pressure-blowing combination method. If the molding process is not properly controlled, it may cause product quality problems such as bubbles, streaks, and stones.
The glass annealing process is a process to reduce or eliminate the thermal stress generated during the glass forming process as much as possible. The glass annealing process is usually divided into five stages: heating and soaking stage, important cooling zone, slow cooling zone, rapid cooling zone, and rapid cooling zone. The control of temperature and time is the parameter index that needs to be paid attention to in each stage.
In order to make the glass have a certain shape and artistic effect, many glass manufacturers will perform post-processing/decoration after annealing. Common post-processing methods include: polishing and engraving, polishing, matte delustering, and colorful glaze decoration, Rainbow decoration, gold-painting decoration, ion diffusion coloring and spray color, etc.
Due to the particularity of the raw materials and processing technology of glass, when it comes to product safety, more people tend to focus on product quality and ignore food safety. The control of the food safety of glass products in national laws and regulations is mainly to control the migration risk of lead and cadmium. However, through the introduction of the above-mentioned production process, we analyzed that if the ingredients of raw and auxiliary materials and the post-processing of products are not properly controlled, they will all have more or less impact on food safety, and the migration risk is not limited to lead and Cadmium these two metal elements. In particular, the application of post-processed colorful glaze decoration, rainbow decoration and gilding decoration greatly increases the risk of metal/non-metallic elements migration in glass. Therefore, we recommend that glass product manufacturers strengthen their awareness of risk management and control of glass products. Conduct risk assessments from time to time. Once risks or potential risks are identified, timely effective measures will be taken to control them to ensure product safety.