imageDaveb, just last evening, wrote in a comment:

I personally see no hope of convincing those who swallow camels and strain at gnats and are predisposed against authenticity, no matter what proofs might come to light! it is too much of a challenge for their world-view. There is no other ancient object for which an explanation can readily be found, but one. And they yet cry “ignorance is no argument of proof!” Make one! Too difficult!

It reminded me of a recent conversation with a friend.

Him:  You say on your blog that you think the shroud is probably real.

Me:  Yes.

Him:  So how do you think the image was created?

Me:  I have no idea. I have never seen a hypothesis I liked.

Him:  Why, because it couldn’t work or because it didn’t fit your faith paradigm?

Me:  I like to this it is simply because it could not work but to be honest it is both.

Him:  So why do you think it is probably real?

Me:  Because I don’t think it is fake.

Him:  Okay but in science that is a fallacy. It is an appeal to ignorance.

Me:  But we are not talking about science.  This is a history of art problem.  We’ve looked. Admittedly, the focus has been European artistic methods and we need to look more for possible methods coming from the Middle East, even maybe Asia and Africa. It has been what, several decades or more than a century that we have been looking for a way that the images could have been manmade. Only a scientist would call that an appeal to ignorance.

Him:  But scientist are working on ways.

Me:  Art forms are not the product of scientific endeavor.  These scientists are more like Van Gogh or Monet than Einstein.  In the end a new art form that probably never existed isn’t going to make the shroud “improbably” real. Showing how it could have been made is not unlike an appeal to ignorance.  It’s art, not science.