Here are some facts from Bell Helwig of Thompson’s Enamel and you can read the whole article here
The absence of a US manufacturer will not detour the enamelist, nor will the use of lead free vitreous enamel decrease the quality level of items produced. The replacement of lead bearing with lead free vitreous enamel will reduce the problem of exposure to lead due to sloppy hygiene and careless work procedures for those using lead free material, but if hygiene and work procedures are not also improved, some other toxic material will probably take its place. Only the individual can improve his or her work habits and so to speak, clean up their act.
As to the specific differences between lead bearing and lead free, their advantages will be listed first. This listing will compare those lead bearing enamels that were produced by Thompson Enamel with the lead free enamel produced by the same company. This comparison does not include or preclude similarities for lead bearing materials produced by foreign manufacture. Such a comparison would probably have an equal number of similar facts among the more common types and colors of vitreous enamel. These differences must be stated in generalizations because no two enamels are alike, unless they are from the same batch. Also, for example, a transparent red made from gold and a transparent red made from cadmium selenium, while both transparent and red and vitreous will always be considerably different under all conditions whether they contain lead or not.
LEAD FREE VITREOUS ENAMEL ADVANTAGES Lead free will by volume weigh less Lead free will be more acid resistant. Lead free may be ever so slightly harder (scratch ability). Lead free may absorb more copper oxide from the metal surface. Lead free may have a larger coefficient of thermal expansion. Lead free will have less water solubility. Lead free will have a slightly higher softening temperature. Lead free opaques will have less color variation due to different firing time/temperature relationship.
LEAD FREE AND LEAD BEARING EQUALITIES Index of refraction. Workability.
LEAD BEARING ADVANTAGES Lead bearing has a slightly decreased skin texture. Lead bearing currently has a greater color selection. Lead bearing has lead arsenate opal colors.
Opinions of My Own
I have been enameling for most of 20 years and using leaded enamels. The first 10 I wore a respirator and had my blood level checked yearly for lead. After the first 10 this got really old and I stopped wearing the respirator. I still have my blood level checked and had no signs of lead. I do not sift, I wash and wet pack. I do my sanding and polishing under running water. And as an enamelist I keep things clean.
I use Bovano Leaded Enamels and Ninimiya Leaded Enamels. I can not replace the blues and greens of all ranges in the Bovano Enamels. And the Minoniya to me are softer enamels. They tend to blend very well. When an enamel is a lower fire enamel the molecules mingle easier. I find hard firing enamels do not. I really noticed this lately as I have been more involved with Plique a Jour. I can see when I finish the grain of glass in my blues are not mingling. So it is not as clear of a transition.