Make sure any lead test swabs you use are ceramic specific. The ones meant for paint can give false results on china.
I have seen some people have problems with certain decal paper brands, or some older decals leaving transfer paper outlines left behind. They don’t have to be a deal breaker. The best way to make this look inconspicuous is to cut them out neatly, and as close to the design as you’re willing to get. It does sound to me like the ones you have need to be fired closer to the 016 mark. If you soaked your decals in water and slid the paper out from between the decal and the pot to apply them, you did it right. If you flipped them face down, they’re upside down, and the paper burning out would have affected their adhesion. The only other troubleshooting thing might be if you didn’t let them dry for at least a few hours before firing them, or if they weren’t well squeegied down. If there was any water or air still trapped under the decal, that’ll mess with your day.
No decal that is fired on top of the glaze at that low temperature is fused fully with the glaze, whether they’re china paint or lustre or etc. They’re cured well, but will wear off with improper use or enough abrasion. If you look at most older china patterns, the surface that contacts the food is often left white, and decoration is on edges or outsides. And you’re not supposed to put it in the dishwasher. Also, it’s not just lead that was used to make some of those colourants. There may be cadmium reds or yellows. And a dish sold as a coaster can easily be repurposed into a dip dish, or prep dish, or holder for dental items. Just something to bear in mind.
I know a lot of the china paints and decals from that time contained lead, I wouldn't be using them on food bearing surfaces. Apart from the lead aspect they are not durable for food surfaces either.
Im vintage and have fired decals 45 years ago-the meduim has to stick to the pot(water based meduims-get wet and let dry)-not curled up and they fire slow to the large cone temp range stated -so 018-016. I used to watch the cone drop (large cone) Before computer kilns . I suggest using large cones and maybe the hotter end cone wise if you see paper outlines. That also could be colorants drifting ?A nice decal pot has NO outlines and no rough textures
I do not think age if stored right will affect them.I have some 45-50 year old ones but have not ues them-butterflies and some ships
Some of the colors may change if they get to hot as well.
Most decals are not food safe anyway.
I fired all on them on horizontal vases mostly in the mid 70s
they all work best on shiny smooth glazes-not matts or semi matts
