The IR and/or NMR examinations will give you the answer about your question. Additionally I would rather say that the observed changes are result of nucleation in your samples.

I. Heat treatment might possibly have lead to the nucleation/growth of nanoclusters/nanocristallites, which actual nature can not be further discussed based on the information available from the enunciate. Nonetheless, it seems that could possibly help to explain the noticed increased absorption in the near infrared (NIR) wavelength range.

II. The effect may possibly be rather small for the current glass and the short heat treatment considered; but we generally expect that Fe(II) species mignt be favoured in its redox equilibrium with Fe(III) ― with a corresponding tendency for the activity of oxygen in its elementary state to also (slightly) increase. Either such molecular or atomic oxygen may have some influence on ionic diffusion, with the possible participation of protons, hydroxyl ions; and where nanoclusters/nanocristallites may possibly also play some kind of role.

III. In principle, a somewhat higher activity of oxygen may, possibly, play a a slight displacement role at Cr(III)/Cr(VI) redox system, perhaps if somehow assisted by sufficient diffusional aid; what may possibly be more effective, close to interfaces/nanointerfaces/inhomogeneities, or even close to the surface. The glass formulation, namely in terms of alkali and alkaline earth oxides, would have to be fully specified, for further discussion.

IV. Based on the query enunciate; it seems that manganese was first introduced as Mn2O3 to the glass formulation, thus as Mn(III). Being that indeed the case; it might, possibly, have been partially oxidized to Mn(IV), during melting, that corresponding to MnO2. Hence, the Mn(III)/Mn(IV) redox system may possibly play a similar role to that of the Cr(III)/Cr(VI) system, while considering further heat treatment, above the annealing temperature.

V. It appears that possible redox processes should be first-hand investigated in what deals with melting. That seems important to better understand the base-glass condition, hence before the mentioned subsequent heat treatment.

VI. It also appears convenient to investigate the visible transmission spectrum of glass samples ― the above disclosed spectrum corresponds to the NIR range.

VII. Direct redox processes involving just the distinct metallic ions above considered, seem, in principle, not likely to significantly occur much below the melting temperature; at least while suggesting what might be some reasonable electrochemical mechanism.

Thank You for detail response, Dr Carlos Araújo Queiroz!

I'll try to tell more clear and detail.

The matrix system is R2O-PbO-SiO2 where R2O is wt 4% Na2O and wt 14% K2O.

This glass system hasn't bulk crystallization. It has only film crystallization after 24 hours heat treatment.

The glass production temperature was 1450 C.

Chromium oxide was added to the glass through K2Cr3O7

Manganese oxide was added to the glass through KMnO4

The literature indicated that in most cases chromium oxide is in the glass in a state of either Cr2O3 or CrO3 and manganese oxide is in the glass in a state of either MnO and Mn2O3. For other oxide states it necessary especially conditions.

Unfortunately, visible spectrum has full absorption in both case.

In my assumption, in addition to the change in transmittance at a 1050 nm wavelength, I consider on the equation of glass solarization [Glastechnische Fabrikationsfehler. By H. Jebsen-Marwedel, Springer-Verlag Berlin, Heidelberg, New York, 1980]:

2 MnO + Fe2O3 = Mn2O3 + 2 FeO

For the reverse reaction, it is necessary to heat the glass to transformation temperatures.

But apart from this equation I haven't met a single confirmation of my assumption.

The IR and/or NMR examinations will give you the answer about your question. Additionally I would rather say that the observed changes are result of nucleation in your samples.

Thank You for answer, Dr Łukasz Gołek!

Do You think that it is the crystals growing can change spectral curve? Did I understand right?

Dear colleagues

I researched IR examinations of both glass sample in powder stan. (Picture 1)

What does mean these results?

Thank You for answer, Mr. Hossam Mohamed Gomaa

I think that it`s one of the reason of my spectral changes in glass too