You need to make sure the press warms up well, it takes a couple of hours for mine to reach a good even temperature.
As I said in the previous reply that tissue is quite a low temperature one designed for all papers, at one time there was tissue for RC papers and a the older higher temperature shellac tissue for FB. However as the lower temperature tissue work with all papers and use a synthetic adhesive they were cheaper and far easier to produce.
I've not dry-mounted a print for over 20 years, but I do remember the very short times needed with the Lion/Hot Press tissue and how much easier it is to use, I've mounted prints with it just using a domestic iron & a sheet of brown paper. I do still have some and will have a go again during the week, I use my press to flatten FB prints.
In addition to Ian's suggestion about warm up time, I remember that solutions to various adhesion problems may lie in finding the right combination of temperature and time, for the tissue and board you are using. Too high a temp can over heat the board and print, which can give the print time to lose adhesion in some areas after removal from the press. I used to remove the mounted print and immediately place it between two large heavy pieces of flat aluminum, under weight. I had two sets of these plates, and wouldn't remove the print till I had placed another print in the other set of plates.
I've never seen this edge problem, but it could be related to this. I haven't dry mounted in some time either.
It is hard to tell the edge profile from your photo; but is there any possibility of the paper being overwashed? I had this happen once with RC paper that an employee processed. The edge of the paper absorbs water and swells permanently. Try mounting a scrap of paper cut from a processed sheet to test this.
This is a FB paper issue, nothing to do with over washing RC papers. It's exactly what I had happen when I switched to this particular company's tissue. Too hot/too long and it does this, it's getting the balance right.
With the older shellac tissues there was no problem but then they need temperatures that can cause major problems with RC papers so newer tissues were developed. Get these too hot and they soak into the mounting card, as the card re-adsorbs moisture the ridge becomes aparent.
Thanks everyone. I will be mounting shortly this morning, I have over 20 prints to do and I will try your suggestions. It very much feels to me like too long or too warm, so I will try limiting those parameters, but perhaps I may need to increase the pressure to prevent the corner de-lamination (when bending after cooling) which got me to try longer/hotter settings.
I really appreciate your wisdom.
Best of luck, let us know how it turns out today.
I suppose I am late with the summary of how it worked out in the end. Well, I reached an impasse. I can either have no dimpling, but the prints will delaminate when I bend the mount board severely at corners, or they will have good adhesion at the price of some edge-long dimples. I think I made the dimples less visible, but I cannot get rid of them entirely. Looking just now at 28 mounted prints I can see that only 6 have almost no dimpling (the edges ever so slightly come up though), while 22 have dimpling. As I have been making notes on the times in the press for each of those, I can only see that what is common to all of the less dimpled prints is that they all (bar one) delaminated after an initial pressing and had to be returned to the press for extra time and overall they were pressed at a lower temperature for longer.
I have been cooling each print between two sheets of glass immediately after pressing for a minute or two, then cooling a little more between mountboards, and testing for adhesion by corner bending.
Maybe I am bending them too much?
I am, overall, at a loss to make sense out of the whole "experiment". I mounted some 40 prints in total. I started with lower temp (80-90˚C, 175-195F below the top protective mountboard, 100˚C, 215F at the platen) and longer times 3-3.5 min. This has led to less edge-long dimpling but far worse adhesion with frequent delaminations at corners, requiring a print to be returned once or twice back to the press. One print went back 3 times before it passed the "bending" test.
Then I decided to increase the temperature progressively at the platen to 121˚C (250F), which was giving temperature of about 90-100˚C (195-215F) under the top protective mountboard (4-ply all the time). Trying a sequence of progressively shorter times clearly failed to activate the Unimount Classic tissue evenly at 45 s pressing time. Time of 1 min 5 s seemed to do the trick, with prints adhering well or requiring one more trip to the press for another 1 min, when they always seemed to adhere. Unfortunately, the dimple was back, perhaps not looking quite like a "valley" but more like gently lifting edges.
I don't know what to try next, but I am keen to try a different tissue to see how it may improve things. I wonder if I can get any ColorMount in Europe, or perhaps I will try one of the HotPress's Archival Grade ones, they are reversible like Seal's BufferMount.
I have also tried other suggestions made here, that is pressing "upside down" with the print under the mount board, the press had been well prewarmed for over a day, and I have been using pre-dried boards and prints.
If anyone wishes to make any other suggestions, I would be grateful.
So on this bending. I have to ask, exactly how much do you bend your boards? Do you think this will happen to your prints? I am assuming that you do this just to make sure the bond is strong, but I would suppose that if the print is on there, I wouldnt expect it to just fall off.
I do flex mine about 3 inches across the whole print when cool. If I don't hear a pop, I let it go. I really don't expect them to either fall off or to be flexed as I expect them to sit in a frame for the rest of their life.
Outside a frame, all sorts of evil can happen to a print. The least of which worry me that someone will bend it over their knee. :)
Rafal - You know, I just read your last post, and you are doing something I've never done in the past (or seen done). You use a much thicker protective layer between the platen and the print than I ever have (4 ply). When I was dry mounting, I used only a sheet of dry mount release paper (sold by Seal at the time) between the print and the platen. This is a waxy feeling paper that is smooth and somewhat glossy on both sides, and will not stick to the tissue. With this, you don't have to allow for temp differential. I actually made a large folder of the release paper, the size of the platen.
I used about 210F for about 25 seconds (3 min + seems very long to me) with good results all the time.
I also pre-heated (to drive out moisture) both the print and the board).
Then tacked the tissue to the print.
While the board was being pre-heated, I trimmed the tissue to the edges of the print with a razor blade.
Then I tacked the print to the board, then put it in the press.
This was with MT-5 tissue and fiber paper. You might try release paper instead, without adjusting the temperature.
George, thanks for your suggestion. I have not tried silicone release paper, but having used a very thin sheet of paper I experienced problems with very uneven heating. I suppose I should try once more with a very short pressing time. Thanks for making this suggestion.