Testing for Lead in Antique Porcelain Teaware

One of the most common questions we receive is from people worried about the use of lead in antique porcelain.   It's a subject we've researched to the best of our abilities over the years and done some amateur testing with home lead test kits.   Our home testing seemed to confirm the information we'd been able to gain from articles read online - that there wasn't a concern from blue and white porcelain but that the main concern came with overpainted glazes.  These were home tests though and we thought it might be useful for tea drinkers if we sent some old porcelain for EDXRF testing for more complete investigation.

Background

First, perhaps it is helpful to define two types of glazing & why lead was/wasn't necessary in each

What is an Underpainted Glaze (Underglaze)?

This technique, most famous in Blue and White ($青花$, qīnghuā) porcelain, involves painting the design directly onto the unfired, dried clay (the "biscuit").

• Process: The artist uses a mineral-based pigment (like cobalt for blue or copper for red). A clear, feldspathic glaze is then applied over the painting, and the piece is fired at high temperature ($1200$°C to $1300$°C).

• Stability: Because the pigment (such as cobalt or copper) is separated by a stable glassy barrier it is not susceptible to leaching

What is an Overpainted Glaze (Overglaze/Enamel)?

This method, common in Famille Rose ($粉彩$, fěncǎi) or Doucai ($斗彩$, dòucǎi), involves applying colors on top of a previously fired, smooth glaze.

• Process: The vessel is first fired at high temperature with its clear glaze. Then, low-temperature "enamels" are painted onto the surface.  These "enamels" consisted of a lead-silica glass powder (as a flux) mixed with metal oxides (for colour).  The piece is fired a second time at a much lower temperature (around $700$°C to $900$°C) to fuse the enamels to the glaze.  The lead-silica flux allowed for the second firing to be done at a lower temperature, vitrifying the enamel decoration while allowing the high temperature clear glaze underneath to remain stable.

• Stability: Because these colours sit on the surface and are fired at lower temperature, they are less chemically stable. Over centuries, these "overpaints" can become matte, scratch easily, or leach heavy metals (like lead or antimony used in the flux) if they come into contact with acidic substances.

Our Testing

While we knew this theory, we thought it good to double check this with some real world testing

We sent 9 items to test, ranging from early-Ming Dynasty, through Qing Dynasty to present day.  

These were the results

Test Reports

Analysis of results

In general there were no big surprises here.  The results were consistent with our expectations of a high lead reading for the overpainted porcelain piece and normal background trace readings in the blue and white underpainted pieces. 

Although not of a safety concern to us due to its physical separation by a clear glaze barrier, one interesting note was the elevated Manganese readings in two pieces, which led us down a rabbit hole of research into the history of sourcing and processing of cobalt throughout the ages.   This high Manganese content is one of the hallmarks of the domestic cobalt used at that time, in comparison to the low manganese cobalt imported from Persia in early Ming or the later refinements of cobalt in the Qing dynasty.  This result didn't appear on all the pieces, we assume due to the variations in the area of the sample tested by EDXRF analysis, which wasn't always on the blue decoration.

We were also happy to see that our modern commissioned Fangxia porcelain teacup had very clean composition in terms of metals.

Safety

 As tea drinkers we must each make our own decisions about the safety of the teas we drink and the teaware we use.  This sample set was limited, but we tried to give a good representation of pieces from different periods.   From our testing of these pieces and focussing specifically on the risks from lead in glazes, we would have some concern about using overpainted porcelain on a daily basis.  There would not seem to be this risk from underpainted glaze pieces such as blue and white porcelain.