Copper has strengthened to a four-week high Friday, with the base metal-related exchange traded funds testing their long-term resistance level, on the revitalized global economy and expanding construction in the U.S.

The iPath Dow Jones-UBS Copper Subindex Total Return ETN (NYSEArca: JJC), an exchange traded note, has gained 5.5% in a little over two weeks and is now testing its 200-day simple moving average. However, JJC is still down 6.5% year-to-date.

COMEX copper futures were slightly positive early Friday, trading around $3.173 per pound.

Through Thursday, copper prices rose for 10 consecutive days, the longest rally since June 2005 and rose 10% from a 44-month low in March, reports Luiz Ann Javier for Bloomberg.

Copper has been strengthening on renewed optimism over global economic growth and an accelerating U.S. recovery, reports Frik Els for Mining.

LME, New York and Shanghai copper inventories declined 50% in 2014, dipping to a six-year low Thursday. Global refined copper market unexpectedly fell to a shortage in March and a 205,000 ton deficit for the first three months of the year, compared to a 206,000 ton surplus for the same period in 2013.

“With stronger demand, we’re seeing inventories decrease, and certainly that’s going to be bullish for prices,” Fain Shaffer, the president of Infinity Trading Corp., said in the Bloomberg article. “Strong economic conditions here and overseas over the short term will drive prices higher.”

Meanwhile, China, the biggest consumer of the metal, has hoarded a record high 115 million tons of the commodity and demand is slowing. China has seen copper prices fall relative to other markets after a crackdown on metal financing. [Copper ETFs Blemished After Chinese Investigation]