Hi Guys The best reults I've had have been from scanning a 35 neg at high resolution(file size 30 meg ++)then doing what ever you want in Photoshop or any other half-decent imaging software, then having a neg made of whatever size you want at a service bureau or place that makes separation negs for printers. I paid less than 10 bucks for an 8x10. MAke sure the output is silver and usually Agfa film is best. Agfa also make a copy film which is supposed to be the best-they call it 36 something.(posted 8378 days ago)Dan Burkholder has an excellent book on making enlarged negs- look for "Dan Burkholder-enlarged negatives" on Google. he sells the book direct. It also contains instructions and special curves for adjusting the contrast on digital negs-absolutely necessary.
They can also be made on modern inkjet printers using a color wash, such as orange( for platinum paper)as a density or contrast enhancer.
Making enlarged positives, then contact printing a dupe neg is OK but you lose sharpness, BUT, this often makes the print look like an old style original platinum print from 1880 or so. Take a look at some Julia Margaret Cameron platinotypes-GORGEOUS!
I finally gave up and am now shooting 8x10 in-camera. A 4x5 camera neg on platinum makes a FANTASTIC contact print and when framed properly can be a real jewel and can sell for thousands. (Elton John says he loves small prints- try sending him one.)
The best developer I have found for negs for Platinum printing are T MAx 400, rated at 200, in split D-23. The instruction book from Palladiio in Cambrifge MA.(Google again)has excellent instructions for all phases of this nonsense.
Cheers