When the tintype came into use in the mid-1850s the conventions of photography loosened. Photographic portraits were now within economic reach to almost everyone. And the exposure time had shortened so that the stiff, unsmiling style of the earlier types was no longer necessary, allowing for a more varied range of facial expressions and poses.
Shortly after tintypes came into vogue enterprising manufacturers produced cameras with multiple lenses that could take multiple photos at a time on one plate. Then the think iron plate was cut into individual photos, usually using tin snips, which may have been where the term tintype originated.
Tintypes were sometimes put into ornate cases like daguerreotypes or ambrotypes, though these weren’t strictly needed to protect the photo since it was more durable and less likely to scratch. As the process got cheaper and more democratized, photographers started simply slipping the finished photos into paper sleeves. If you’re lucky enough to have cases or sleeves left with your tintypes information can be gleaned from these to narrow a date. There may be a photographer’s logo or name printed on them, or even handwritten information.
Check the back of the tintype as well. During a brief two-year period, from August 1, 1984 to August 1, 1866 the federal government levied a tax on photographs and photographer were required to attach a revenue stamp to the back of the tintype and initial and date the stamp.
There might also be information available from decorative elements on the paper sleeve. During the Civil War the sleeves were sometimes embossed with patriotic symbols such as stars and stripes.