1. Tokugawa Ieyasu's Glasses and the Sea Turtle Shell Said to Be Used for Them
In the Kunozan Toshogu Shrine in Shizuoka Prefecture, a pair of glasses owned by Tokugawa Ieyasu (1542–1616) in his later years is preserved as an Important Cultural Property. According to an explanation by the Ministry of Land, Infrastructure, Transport and Tourism, they are made of tortoiseshell, which is processed sea turtle shell. The explanation notes that he likely used them by resting them on his nose or supporting them with his hand [1].
Interestingly, the explanation does not mention temples, the arms that hook over the ears. This suggests they were quite different tools from modern glasses. Records also show when glasses first arrived in Japan. In 1551, the missionary Francis Xavier is said to have presented glasses to Ouchi Yoshitaka, the feudal lord who ruled Suo (now Yamaguchi Prefecture) [2]. Glasses themselves are said to have been invented in Italy in the late 13th century [2].
More than 400 years have passed since then, and frame materials have changed significantly. What did they change into?
2. How Frame Materials Changed from Flammable Celluloid to Acetate
After the era of Ieyasu, one material widely used for frames was celluloid. It is a type of synthetic resin made mainly from nitrocellulose (guncotton) and camphor [3]. Its major weakness was that it was extremely flammable. It could even catch fire from heat generated by friction, and it was prone to damage from light [3]. These are concerning properties for an item placed right next to your face.
Because of this, there is a trend toward using acetate (acetyl cellulose) instead of celluloid, as acetate is less flammable [3]. Some explanations state that most modern glasses are made of acetate [4].
There is a town in Japan that is the country’s largest eyeglass-frame production area: Sabae in Fukui Prefecture.
3. Why Sabae, a Farming Town, Started Making Glasses as a Winter Job
In Sabae (Fukui Prefecture), winter is the agricultural off-season. Therefore, it is said that Masunaga Gozaemon started making glasses as a winter side job in 1905 (Meiji 38). Stories also tell of craftsmen being invited from Osaka [5].
From there, glassmaking in Sabae spread, and it is explained that it became the number one production area in Japan in 1935 [2]. Furthermore, an article from nippon.com (written around January 2012) states that the production volume of eyeglass frames in Sabae City reaches about 96% of domestic manufacturing [5]. Note that these figures change from year to year, so this is not the latest value.
A job for the agricultural off-season became the top production area in about 30 years. This momentum led to a challenge involving a difficult metal.
4. How Sabae Developed Lightweight, Strong Titanium Frames in the 1980s
In the 1980s, Sabae is described as establishing the manufacturing technology for eyeglass frames made of titanium, which is light and strong, for the first time in the world [5]. Titanium is a metal that is difficult to process. Because sources differ on the exact year (1981 or 1983), we will stick to "the 1980s" here.
Titanium is resistant to rust, strong, light, and relatively expensive. It is also said to be often used in frames for seniors [4]. The American Academy of Ophthalmology (AAO) lists titanium as a material that is "very strong, light, and rust-resistant" and less likely to cause allergies [6].
Other metal frames include stainless steel, aluminum, and monel, an alloy that may contain nickel. Plastic options include cellulose acetate and nylon-based materials [6]. Frame materials are not limited to one type; they can be chosen based on lightness, price, color variety, and skin compatibility. Next, let’s look at the material of the lenses, the part used for seeing.
5. Why Plastic Lenses Replaced Glass Lenses as the Main Material
Lens materials are broadly divided into glass and plastic. Glass has good optical properties and is resistant to scratches. However, it is weak against impact and tends to be heavy [4].
Plastic lenses are characterized by being hard to break and light. There is an explanation that plastic lenses are widely used today [4].
There are also types within plastics. CR-39 is a material developed in 1947 that weighs about half as much as glass. Polycarbonate appeared in the early 1970s; it is even lighter and stronger against impact, so it is said to be suitable for children’s and sports glasses [7]. So, why does the thickness change depending on the material, even with the same prescription and lens design?
6. How Lens Material and Refractive Index Make Lenses Thinner for the Same Prescription
Lenses are tools that bend light to focus it at the back of the eye. The strength of bending light is expressed by a number called the "refractive index." Materials with a higher refractive index can be made thinner for the same prescription and lens design [7].
In the case of plastics, the refractive index of CR-39 is 1.498, while polycarbonate is 1.586 [7]. The difference is 0.088, meaning polycarbonate is about 5.9% higher. Since a higher refractive index allows for thinner lenses, polycarbonate is a material suited for thinness in addition to lightness and impact resistance.
Light, hard to break, and thin. Because of this combination of properties, we can now choose lenses according to their purpose.
7. How to Observe Differences in Frame and Lens Materials With Everyday Glasses
You can observe glasses belonging to your family or yourself on a desk. Ask the owner for permission before handling the glasses.
First, place a few pairs of family glasses on your palm to feel the difference in weight. You might notice a difference in lightness between thin, metal-like frames and thick, colored frames.
Next, look at the lenses from the side and compare the thickness of the edges. However, since prescriptions vary from person to person, you cannot determine if the difference in thickness is due to the material alone. Enjoy this as practice in noticing differences.
If you want to learn more, you can read the explanation of Ieyasu’s glasses in the Ministry of Land, Infrastructure, Transport and Tourism database [1], and the list of frame materials in the AAO explanation [6]. If you look at the inside of the frame of the glasses at home, you may find the material name or model number written there.