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Introduction Procedure List of Contents Appendices Photo/Map Sitemap History

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Construction of the Capsule

The background

The largest time capsule in existence prior to 1970 was built by the Westinghouse Electric and Manufacturing Company of the United States of America in 1938 and buried on the site of the New York World's Fair. Its capacity was approximately 40 litres. For the Time Capsule Expo '70 project it was felt that the capsule should have much greater capacity, at least ten times greater than the Westinghouse capsule, so that it could contain a high proportion of full-sized objects and scientific samples.

The technical challenge was considerable: to construct a monolithic, hollow container capable of withstanding external stress (Japan is subject to frequent earthquakes of varying magnitude) while being impervious to humidity, oxygen, carbon dioxide and ultraviolet light. This was no easy task.

The basic design

During their initial discussions, the technical committee and the Research and Development Centre decided that the capacity of the capsule should be 500,000cm³ (500 litres) and that it should be spheroid in shape. A sphere has the best surface area/capacity ratio of any hollow form and it is the most stable shape from the standpoint of structural dynamics. It was also established at an early stage that the capsule should have an inner and outer lid, three strong carrying lugs attached to the body and three sturdy feet. The thickness of the body wall would range from 35mm at the top and 70mm at the base. Unladen weight, including the lids, would be 1.74 metric tons.

Choosing the right material

The material of which the sphere would be made had to meet the following conditions :

  1. Great constitutional strength and high tenacity against external stress.
  2. Resistance to rust and corrosion.
  3. Stable structural and chemical characteristics over a wide range of ambient temperatures.
  4. Resistance to stress during welding.
  5. Ideally the material should have been developed in Japan.

After exhaustive studies of the various materials available, a special stainless steel designated NTK-22AT was chosen for the capsule. NTK-22AT is an austenitic stainless steel containing 22% nickel, 20% chromium and very little carbon. A titanium additive inhibits progressive crystalline phase transformation caused by the diffusion of carbon. In other words, NTK-22AT is a high-grade alloy which is both extremely strong and highly resistant to corrosion.

However, NTK-22AT was developed originally as a material suitable for forming into rods and plates. Its practical applications include artificial heart valves, pins and plates for use in orthopaedic surgery and components for high-quality watches.

In the case of the capsule, the metal would need to be formed into a large, hollow sphere.

Although NTK-22AT is known to be stable during mechanical processing – that is, forming and bending – it was decided to keep processing to a minimum and manufacture the capsule by casting. This decision presented many difficult technical problems in view of the size and shape of the container and the need to preserve the characteristics of the alloy. Ultimately, the technical committee and their advisers developed a special method of reverse-casting which proved to be entirely successful. A test capsule was cast in April, 1969, and the chemical composition of the cast body was as follows:

Weight %

  C Si P S Mn Ni Cr Mo Cu Ti Fe
Composition of cast body 0.02 0.3 ∼ 0.8 0.01 ∼ 0.02 0.01 ∼ 0.02 1.5 ∼ 1.9 22.0 ∼ 22.5 20.0 ∼ 20.5 2.06 ∼ 2.24 1.1 ∼ 1.2 0.12 ∼ 0.64 Rest
Target value ≤ 0.03 ≤ 1.5 - - ≤ 2.0 21 ∼ 23 19 ∼ 21 1.75 ∼ 2.75 1.0 ∼ 2.5 ≥ 0.30 Rest

To ensure a faultless structure, each of the capsules was given a thorough examination after casting, using Ir192 gamma rays.

Sealing and welding

Many of the objects destined to go into the capsule were made of materials that deteriorate progressively in the presence of oxygen and carbon dioxide. For this reason, air would be extracted from the interior of the sealed capsule and replaced with argon gas. The retention of this inert replacement atmosphere was one factor in determining the best method of sealing the capsule; the other factor was the prevention of corrosion.

NTK-22AT stainless steel is known to be highly resistant to the dispersion of carbon. Clearly, it was necessary to find a method of sealing the capsule which would have equally good characteristics in this respect. Many conventional methods were unsuitable: plastic packing, for example, is highly suspect as a source of carbon. Metal packing may produce intra-crystalline corrosion. Both types of packing may set up stress and metal fatigue. Bolts are also unreliable due to the possibility of corrosion and metal fatigue.

After careful consideration of the long and short-term efficiency of several different materials and methods, it was decided to seal both inner and outer lids by electric welding. To facilitate welding and eventual remove of the lids, both lids were formed with a flange and groove-welded. The chemical composition of the electrode was similar to that of NTK-22AT stainless steel.


The capsule was given an aventurine finish by shot-blasting followed by sandblasting. Finally, the name plate moulded into the side of the body was hairline-finished and engraved : EXPO '70 TIME CAPSULE.

The second capsule (the first being the test capsule) was completed on November 28th, 1969, in readiness for display in the Matsushita Pavilion at Expo '70. Three more capsules were completed by the end of May, 1970. The manufacturing sequence, numbering and final destination of each capsule body was as follows:

Order of Manufacture Final Designation Final Site
1st Test Capsule Vertical cross-section displayed at Osaka City Museum
2nd Exhibit Matsushita Pavilion, Expo '70: later donated to City of Osaka and displayed at Osaka City Museum
3rd Time Capsule No.2 Buried adjacent to Osaka Castle ( upper)
4th Time Capsule No.1 Buried adjacent to Osaka Castle ( Lower)
5th Exhibit Permanent exhibit at Matsushita House of History, Kadoma, Osaka

In the interval between the manufacture of the test capsule and the sealing of the capsules destined for burial, Nos. I & 2, in December, 1970, the technical committee concentrated on the treatment and storage of the diverse objects and records that made up their contents.

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The contents of this site are excerpted from THE OFFICIAL RECORD OF TIME CAPSULE EXPO'70(March 1975). Please note that company and organization names may differ from those of the current ones.

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