Harrington A. Lackey writes in Ezine about The Sudarium of Oviedo: The Other Linen in Christ’s Tomb:
According to John’s Gospel, when the tomb was found empty on the third day, not only was the linen that covered His body found, but there was a "napkin, that was upon Jesus’ head, separate from the linen cloths, but rolled up in a place by itself." (20:7). The original Vulgate Bible calls it a "sudarium" which translates to "sweat cloth" in Latin. This piece of linen measures about 2.5 feet length * 1.8 feet height.
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Jesus could not breathe because His arms were stretched out and up on the cross. Also, His feet were nailed, so He could not pull himself up in order to breathe. As a result, His lungs filled with edema which caused asphyxiation. There are also stains of blood surrounding these two large ones. All of this blood covered the entire face due to the blood from the crown of thorns still on His head. There was so much blood on His face that when His body was taken down from the cross and set down inside the tomb, the cloth was mostly blood-soaked. The blood on Jesus face covered about half of the cloth’s surface. It was tossed aside like a dirty rag before preparing Him for burial.
There still is some confusion about the Sudarium, with some skeptics arguing that a sudarium would have prevented the image on the Turin Shroud and others arguing that the burial shroud would not have been used to cover Jesus’ head at all because of the use of a sudarium.