Pure copper is commercially refined copper with at least about 99.3% copper, and the grade — not the label — tells you how pure it really is.
A copper cup stamped “100% copper” can still contain oxygen, sulfur, and trace metals, because no ordinary commercial copper product is literally 100.000% copper. What the phrase usually signals is copper metal rather than plated steel or a brass alloy. Knowing the difference matters: the trace fraction left over is what changes how a material conducts, welds, and holds up over time.
The sections below cover the grades US suppliers actually sell, what a purity percentage permits, and how to verify a claim before you buy.
What Do The Main US Copper Grades Actually Specify?
The most common North American grades are C11000 electrolytic tough pitch copper, C10100 oxygen-free electronic copper, and C10200 oxygen-free copper, and each carries a different minimum copper content and oxygen limit. The UNS designation and the governing ASTM standard control the specification — not the marketing copy on the package.
| Grade And Name | Minimum Copper | Typical Use |
|---|---|---|
| C11000, electrolytic tough pitch (ETP) | 99.90% | Wire, cable, busbars, windings |
| C10100, oxygen-free electronic (OFE) | 99.99% | Waveguides, accelerators, glass-to-metal seals |
| C10200, oxygen-free (OF) | 99.95% | Vacuum and high-temperature service |
| C10300, C10400, C10500, C10700 | Varies by UNS spec | Controlled residual or phosphorus-bearing grades |
| General “essentially pure” copper | 99.3% or higher | Broad commercial and decorative stock |
| Highly refined grades | Approaching or above 99.99% | Demanding electronic and vacuum work |
The Copper Development Association’s grade reference lists these wrought coppers by UNS number, and ASTM B170-24 sets the oxygen ceiling for the oxygen-free grades at no more than 0.0010%, or 10 parts per million.
What Does A Purity Percentage Actually Permit?
A claim of 99.9% copper means copper accounts for at least 99.9% of the material by mass, leaving up to 0.1% for oxygen and other elements — and that leftover fraction is not trivial. A 99.99% claim tightens the allowance to 0.01%. Those few hundredths of a percent can shift welding behavior, brazing results, conductivity, ductility, and corrosion resistance.
Treat “100% copper” as a commercial description rather than proof of chemical purity. Solid copper can still carry oxygen, sulfur, phosphorus, silver, or iron. Copper-plated steel, brass, bronze, and copper-nickel alloys are not pure copper at all, even when the surface looks the part.
Why Does Oxygen Content Matter So Much?
Oxygen is the dividing line between the two families. ETP copper deliberately contains oxygen and is the practical pick for ordinary conductors. Oxygen-free copper is produced under controlled non-oxidizing conditions and is preferred where welding, brazing, vacuum service, or high-temperature processing could turn that oxygen into processing defects.
Oxygen-free and oxygen-free electronic are not interchangeable. The required oxygen limit, conductivity, mechanical condition, dimensions, and end-use specification all have to be checked against the job.
Conductivity gets misread just as often. A rating above 100% IACS does not mean the material is more than 100% copper — it means conductivity exceeds the historical reference value. ETP copper commonly measures about 100 to 101.5% IACS, and oxygen-free grades can land above that figure. A conductivity number is not a purity percentage.
Higher purity also is not automatically better. For a copper drinking cup or a decorative piece, solid copper of a sound commercial grade does the job, and the grade name on the listing tells you more than the percentage headline. If you are comparing drinkware rather than mill stock, this roundup of tested 100% pure copper cups covers what the labels on finished products actually mean in practice.
How Do You Verify A Copper Purity Claim?
Ask for the exact UNS designation and the governing standard, then check the stated minimum copper content and maximum oxygen or impurity limits against them. For critical applications, request a mill test report or certificate of analysis. The Copper Development Association’s wrought copper grade listings let you match a UNS number to its specified composition.
Never infer purity from color, weight, a magnet test, or the phrase “100% copper.” All four fail on plated and clad material. If the use is electrical, vacuum, brazing, or high-temperature, confirm the grade is compatible with the equipment and the joining process before you commit.
Common Questions
Is a copper cup labeled 100% copper actually pure?
It is solid copper rather than plated steel or an alloy, and that is usually what the label means. Commercial copper is refined to at least about 99.3% and often higher, so a small fraction of oxygen and trace elements remains. Ask the seller for the UNS grade if the exact composition matters to you.
Does a higher copper percentage always mean better quality?
No. Electrolytic tough pitch copper at 99.90% is the standard choice for ordinary electrical work, while oxygen-free grades serve vacuum, electronic, and specialized joining needs. The right grade depends on the application, and a higher number is not automatically the better fit.
What does a rating above 100% IACS mean?
It means the material conducts better than the historical annealed copper reference value, not that the copper content exceeds 100%. Oxygen-free grades can measure above 100% IACS while ETP copper typically lands around 100 to 101.5%. Conductivity and purity are separate measurements.
Sources
- Copper Development Association. “Wrought Copper Grades” Lists US copper grades by UNS designation and composition.
- ASTM International. “ASTM B170-24, Standard Specification for Oxygen-Free Copper” Defines oxygen-free grades and the 0.0010% oxygen limit.
- Copper Development Association. “Introduction to Copper: Types of Copper” Explains ETP versus oxygen-free copper and their applications.